Hot Water System Smells: What’s Normal, What’s Not, and What to Do (Sydney Guide)

If you live in Sydney, you’ve probably noticed the way hot water “behaves” changes through the year — especially in homes with gas hot water systems. Winter mornings can make any odd smell feel stronger, coastal humidity can turn cupboards musty, and older homes can add their own plumbing quirks.

The tricky part is this: some smells are annoying but harmless, while others are an early warning sign of a leak, bacteria, corrosion, or a safety issue. This guide helps you sort “normal-ish” from “not normal” using practical, homeowner-safe checks you can do without pulling anything apart.

What counts as “normal” for hot water smells?

A mild, brief smell that appears only for a few seconds when you first run the tap can be normal in some situations, especially if:

• The house has been empty for a few days, and water has sat in the pipes
• It’s only at one tap (often the tap aerator or that one section of pipework)
• The smell disappears after the water runs for 10–30 seconds
• There are no other warning signs (no discolouration, no new noises, no dampness around the unit)

Even if it’s “normal”, it’s still worth noting what type of smell it is and whether it’s changing over time.

What’s not normal

Treat it as “not normal” when the smell is:

• Persistent (every time you run hot water)
• Getting stronger over days/weeks
• Accompanied by discoloured water, metallic taste, sediment, or pressure/temperature issues
• Noticeable around the hot water unit itself (cupboard/garage/balcony)
• Similar to gas, burning, or chemicals

Q: Why does hot water smell stronger than cold water?

Hot water warms up odours and releases dissolved gases more readily. That means a mild issue can “announce itself” much more when the tap is hot, even if the root cause is in the water or tank rather than the tap.

First 10 minutes: the safety check (especially for gas-like odours)

Some smells need urgent caution. If you think the odour is “gas-like” (or you’re unsure), treat it seriously.

Do this straight away:
• Don’t use matches, lighters, or anything that could create a spark
• Don’t turn electrical switches on/off in the affected area if you can avoid it
• Open doors and windows for ventilation if it’s safe
• Leave the area and move everyone outside if the smell is strong or you feel unwell
• If there’s immediate danger, call emergency services (000 in Australia)
• Contact your gas distributor’s emergency line (usually listed on your gas bill)

For consumer-facing safety information on gas odourisation and what to do, see the NSW Government’s guidance: Gas safety requirements and consumer rights.

Q: Is a “rotten egg” smell always a gas leak?

Not always. Rotten egg can also come from the hot water itself (often linked to hydrogen sulphide from bacterial activity inside storage tanks). The key difference is where the smell shows up:

• If it’s mainly from the hot tap water (especially the water itself), it can be a tank/water issue
• If it’s strongest in the air around the unit, meter, or gas appliances, treat it as a possible gas issue

When in doubt, default to safety.

The simplest way to narrow the source (no tools required)

Before you guess the cause, run these checks.

1) Hot vs cold test

Run cold water at the same tap for 10 seconds, then hot water for 10 seconds.

• Smell only on hot: more likely tank/water-heating related
• Smell on both: more likely water supply, drain gases, or tap/pipe area issue

2) One tap or many?

Check a second tap (kitchen vs bathroom is ideal).

• Only one tap: often the tap aerator, a local pipe run, or that fixture’s drain/trap area
• Multiple taps: more likely the hot water unit/tank, or whole-of-house water quality

3) “Water smell” or “cupboard smell”?

Stand near the hot water unit (without touching it) and compare:

• Does the water coming out smell?
• Or is the smell strongest in the cupboard/garage/balcony air?

That distinction matters. Musty cupboard smells often point to moisture and ventilation, not the water itself.

Q: What if the smell is worse first thing in the morning?

That can happen when water sits in pipes overnight. It’s a clue, not a diagnosis. If it clears quickly and only appears at one tap, it leans “local plumbing”. If it happens at multiple taps and persists, look further upstream.

Smell type guide: what it usually means and what to do

Below are the most common hot water smell complaints and how to handle them safely.

Rotten egg/sulphur smell (eggs, swampy, “sulphury”)

What it often means

A sulphur/rotten egg smell in hot water is commonly linked to hydrogen sulphide gas. One common pathway is bacterial activity in a storage tank (especially when conditions suit it), sometimes interacting with the tank’s sacrificial anode.

It can be more noticeable in:
• Storage hot water systems
• Homes where hot water is used less often (guest bathrooms, vacant periods)
• Warmer months (but it can happen any time)

What you can do safely

• Flush the hot tap for a short period to see if the smell clears
• If it’s only at one tap, clean the tap aerator (it can trap odour-causing build-up)
• Note whether the smell is only hot, and whether it’s worse after non-use

What to avoid

• Don’t start mixing strong chemicals or DIY “tank treatments” if you’re not confident. Incorrect dosing or incompatible products can create hazards and damage components.

When it’s time to get help

If the smell persists across multiple taps, keeps returning, or is accompanied by discoloured water or unusual noises, it’s a good sign the system needs inspection/maintenance. If the smell keeps returning and you’re weighing up longer-term fixes, it can help to compare gas hot water options so you understand what types are commonly used in Sydney homes.

Q: Can a rotten egg smell in hot water make you sick?

Most complaints are about odour and taste rather than illness, but any persistent odour change is worth addressing. If anyone in the household is medically vulnerable, take a conservative approach and seek professional advice rather than living with it.

Musty/mouldy/damp smell around the unit (cupboard, garage, balcony)

What it often means

In Sydney’s humid spells (and in coastal areas), “musty” is frequently about moisture rather than the hot water itself:

• Small leaks or weeping valves that dampen a cupboard
• Condensation in poorly ventilated hot water enclosures
• Mould/mildew growth in cabinetry or plasterboard
• Drain/trap issues nearby (sometimes mistaken for hot water smells)

This is common in apartments and terraces with hot water systems tucked into tight cupboards.

What you can do safely

• Look (don’t touch) for damp patches, bubbling paint, swollen skirting, or mould specks
• Improve ventilation: open the cupboard periodically and ensure vents aren’t blocked
• Wipe obvious condensation and monitor whether it returns
• Check whether the smell is present even when you’re not running hot water

When it’s time to get help

If there’s ongoing dampness, visible mould, or any sign of leaking, it’s worth getting it assessed sooner rather than later. Moisture problems can damage cabinetry and walls quickly.

Q: Why does it smell musty only when the hot water runs?

Heat and steam can “activate” odours in a cupboard, especially if there’s moisture. The water may be fine, but the environment around the unit isn’t.

Metallic smell or rusty taste (coins, iron-like)

What it often means

Metallic odours can point to:
• Corrosion somewhere in the hot water path
• Ageing components inside a storage tank
• Rust or scale in older pipework (common in older Sydney homes)

You may also notice:
• Discoloured water at the start (yellow/brown tinge)
• Sediment or grit in the bath or sink

What you can do safely

• Run the hot tap for 30–60 seconds and see if it clears
• Check another tap to see if it’s local or across the house
• Avoid drinking water that tastes metallic until you’ve identified the cause (use cold water for drinking in the meantime)

When it’s time to get help

If the metallic smell persists, returns daily, or is paired with discolouration, it’s usually a sign the system needs a closer look. If you’re unsure what modern systems look like (or what might suit your household size), it’s useful to explore gas hot water models while you’re narrowing down the cause.

Q: Is a metallic smell an emergency?

Usually not immediate in the way a gas smell is, but it shouldn’t be ignored. Persistent metallic odour can signal deterioration that gets worse over time.

Burning smell (plastic-like, electrical, “hot dust”)

What it often means

A burning smell is more concerning because it can relate to:
• Overheating components
• Electrical issues (for systems with electrical elements or controls)
• Dust burning off (sometimes mild, brief, and seasonal)

What you can do safely

• If it’s brief and happens after a long period of non-use, it may be dust burning off a warm surface
• If it’s persistent, strong, or worsening, stop using hot water and treat it as a safety issue

When to escalate

Persistent burning smell isn’t a “wait and see” situation. If you suspect overheating or an electrical issue, arrange a check promptly.

Q: What if the smell is only when the unit is heating?

That timing can be a clue that something is warming up that shouldn’t be (or warming too much). Don’t open panels or attempt DIY internal checks.

Sewer / “rotten drain” smell (often mistaken for hot water)

What it often means

Sometimes the hot water isn’t the culprit. Running hot water can change airflow and release drain gases, especially if:

• A floor waste dries out
• A sink trap has issues
• A bathroom has poor ventilation
• There’s a drain blockage starting to form

What you can do safely

• Smell-test near the drain (not just the tap)
• Run water in rarely used floor wastes/sinks to refill traps
• Check if the smell appears with cold water too

Q: If it’s drains, why do I notice it during showers?

Warm steam and airflow can pull odours out of drains more strongly, and bathrooms are enclosed spaces. It’s not always the hot water system.

Sydney-specific scenarios worth knowing

Apartments and strata setups

In some buildings, hot water may be centralised or located in tight service cupboards. If the smell seems “in the building” rather than the water, it may involve shared infrastructure. Keep notes:

• Where you smell it (tap water vs cupboard air)
• Which fixtures are affected
• When it happens (morning only, after long non-use, every time)

That info helps strata and trades narrow it down quickly.

Coastal humidity and mould risk

Eastern suburbs and bayside areas can see higher humidity. A cupboard that’s “fine” in winter can become musty in summer. Ventilation isn’t glamorous, but it prevents repeated odour issues.

Older homes and mixed pipework

Inner-west terraces and older freestanding homes often have a mix of old and newer plumbing sections. That’s why one bathroom can smell odd while the kitchen doesn’t.

What to do next: a simple decision checklist

Use this to choose the safest next move.

If it smells like gas (or you’re unsure)

• Prioritise ventilation and leave the area if the smell is strong
• Avoid ignition sources and unnecessary switching
• Call 000 if there’s immediate danger
• Contact your gas distributor’s emergency line

If it’s rotten egg/sulphur in the water

• Confirm hot-only vs hot-and-cold
• Check if it’s one tap or many
• Clean the tap aerator if it’s local
• If it persists across the house, plan an inspection/maintenance assessment

If it’s musty around the unit

• Look for moisture signs
• Improve ventilation
• If dampness continues, address it quickly (moisture damage spreads)

If it’s metallic/discoloured

• Check multiple taps
• Note colour/taste changes
• If persistent, get it assessed (corrosion and sediment don’t usually fix themselves)

If it’s burning/chemical

• Stop using hot water if the smell persists
• Treat as a safety issue and arrange a check

If the issue turns out to be ongoing and you’re planning ahead, you can view a gas hot water range to see the common setups people use across Sydney homes. 

FAQs

Why does only my hot water smell, not the cold?

Hot water releases dissolved gases more easily and can amplify odours caused by bacterial activity, tank conditions, or heated pipework. If cold never smells and hot always does, it points away from the mains supply and more toward the hot water pathway.

Why does the smell happen only at one bathroom tap?

That often indicates a local issue: a dirty aerator, a section of pipe that holds water longer, or even a nearby drain/trap smell that you’re associating with the tap. Compare with the kitchen/bathroom to confirm.

Can my hot water system cupboard smell bad even if the water is fine?

Yes. Dampness, condensation, minor leaks, and poor ventilation can create musty odours in enclosed cupboards. The smell may spike when you run hot water because the area warms up and humidity rises.

What smell should make me act immediately?

Gas-like odour, strong burning smell, or any smell paired with dizziness, headaches, or nausea should be treated urgently. If you’re unsure, always choose the safer option and escalate.

Will flushing the taps fix bad smells?

Running the tap can clear odours caused by stagnant water in pipes, especially after the home’s been vacant. If the smell returns repeatedly across multiple taps, flushing alone usually won’t solve it.

Is it safe to use hot water if it smells like rotten eggs?

If it’s clearly a water odour (not a gas odour in the air) and there are no other warning signs, it’s often not an immediate emergency, but it should be addressed. Persistent sulphur smell is a “fix it” problem, not a “live with it” problem.