Why Does My Hot Water Smell Like Rotten Eggs?

You turn on the hot tap and the bathroom fills with a smell like rotten eggs or sulphur. The cold tap is fine. Nobody else in the street is complaining. It is just your hot water, and it is getting worse.

The short answer is that the smell is coming from inside your hot water tank, not from Melbourne’s water supply. It is a reaction between the water, bacteria naturally present in it, and a metal rod sitting inside the tank. It is not dangerous to breathe, but it is telling you something about the condition of the unit, and in older systems it is often the first warning that the tank itself is on the way out.

Everything below assumes it is the water.

What Actually Causes the Rotten Egg Smell

Three things have to be present at once, and a storage hot water tank supplies all three.

1. Sulphate in the water

Melbourne’s water is high quality and perfectly safe to drink, but like all water, it carries dissolved minerals, including small amounts of sulphate. On its own, sulphate has no smell at all.

2. Sulphate-reducing bacteria

These are harmless, naturally occurring bacteria that find their way into every water system. They cannot survive in any real numbers in the chlorinated cold supply, but a warm, dark, low-oxygen storage tank is close to ideal conditions for them.

3. A magnesium sacrificial anode

This is the part most people have never heard of, and it is the key to the whole thing.

Inside every storage hot water tank is a long metal rod called a sacrificial anode. Its entire job is to corrode instead of your tank. Steel tanks full of hot water rust, so manufacturers fit a rod made of a more reactive metal, usually magnesium, and the corrosion attacks that rod rather than the tank wall. It is a deliberate sacrifice, which is where the name comes from, and it is the single reason a steel tank full of hot water lasts ten years instead of two.

As the magnesium anode corrodes, it releases electrons. The bacteria use those electrons to convert the sulphate in the water into hydrogen sulphide gas, and hydrogen sulphide is what rotten eggs smell like.

So in a strange way the smell is proof your anode is doing its job. The problem is what it tells you about how hard that reaction has been running, and what happens to the tank once the anode is finally gone.

The Four Stages, and Why Stage 2 Is the One That Matters

The reason to act at stage 2 rather than stage 3 is simple. At stage 2 you are buying a part. At stage 3 you are buying a hot water system.

Worth knowing about stage 3, because it catches people out: the smell fading is not good news on its own. If the smell eases and the hot water starts running rusty or discoloured, that is usually the anode being gone rather than the problem being solved. A unit that has reached that point and is also leaking from the bottom has told you everything you need to know.

Why It Is Worse in Some Melbourne Homes Than Others

A few situations reliably produce this smell, and we see all of them.

• Older storage units. The longer the anode has been working, the further along the reaction is. Most Rheem, Dux and Vulcan storage tanks past the seven or eight year mark are candidates.
• Holiday homes, rentals between tenancies, and spare bathrooms. Water that sits still and warm for weeks gives the bacteria uninterrupted conditions. The smell is often worst on the first shower after a property has been empty.
• Households that turned the thermostat down. Lowering the temperature to save on running costs makes the tank a friendlier environment, not a cleaner one.
• Homes on tank or bore water. Less common across metropolitan Melbourne, but on the fringes it shifts the water chemistry and makes the reaction more likely.

What Not to Do

• Do not pour bleach into the tank. It is the most commonly suggested home remedy,y and it is a poor idea. Bleach is not formulated for a pressurised steel vessel;l, it can attack the tank lining and fittings, and at best it masks the problem for a few weeks while the underlying reaction continues.
• Do not drain and flush it yourself. A storage tank holds a large volume of scalding water under pressure. Draining one involves isolating the supply correctly, managing the discharge safely, refilling without airlocking the system, and getting any of that wrong is how people end up burnt or with a unit that will not restart.
• Do not turn the thermostat down to reduce the smell. It may help briefly, and it makes the underlying situation worse. Stored water needs to be held hot enough to suppress bacterial growth, which is why the stored temperature is set where it is. The tap temperature is then controlled separately so nobody gets scalded.
• Do not simply live with it. The smell is a status report on the anode. Ignoring it does not stop the clock; it just means you find out at stage 3 or 4 instead of stage 2.

What a Plumber Actually Does About It

1. Confirm it is the tank, not the supply or a gas issue. Hot tap only, cold tap clean. That takes a minute and rules out the two things that would change everything.
2. Inspect the anode. The rod comes out, and its condition tells you which stage you are at more reliably than the smell does. A rod largely consumed is the answer to the whole question.
3. Replace the anode with a different material. A magnesium anode is the standard fit and the one that feeds this reaction. Changing to another anode material addresses the cause rather than the symptom, and the right choice depends on the unit and the local water, which is a decision for the plumber on site.
4. Flush the tank. This clears the accumulated sediment the bacteria shelter in, and it is done properly, with the supply isolated and the discharge managed.
5. Check that the stored temperature is correct. If a previous owner or tenant has wound the thermostat down, restoring it is part of the fix rather than an optional extra.

Repair or Replace? The Honest Answer

Age decides it more than anything else.

A unit under roughly eight years old with a smell and otherwise clean water is a straightforward anode job, and it is well worth doing. One past ten years, where the anode is gone and the water has started running discoloured, is a different conversation. Fitting a new anode into a tank whose wall has already begun corroding buys months rather than years, and the honest advice at that point is to put the money toward a replacement system instead. If you are replacing, a heat pump is worth considering, and there is a Victorian rule about replacing end-of-life gas systems that is worth knowing before you decide.

What we will not do is sell you a tank when an anode would have done, or fit an anode into a tank that is already finished.

Frequently Asked Questions

Is smelly hot water dangerous to shower in?

Hydrogen sulphide at the concentrations produced in a domestic tank is unpleasant rather than harmful, and showering in it is not a health risk. What it signals about the condition of your tank is the part worth acting on. If the smell is in the air rather than in the water, that is a different situation entirely and should be treated as a possible gas leak.

Why does only the hot water smell and not the cold?

Because the reaction happens inside the storage tank. The cold supply is chlorinated, moving and cool, which is a hostile environment for the bacteria involved. Your tank is warm, dark and still, which is close to ideal for them. A smell on the cold tap as well points to something other than the hot water system.

Will flushing the tank fix it permanently?

Usually not on its own. Flushing clears the sediment the bacteria live in and often reduces the smell for a while, but if the magnesium anode is still in there feeding the reaction, it tends to return. Flushing plus an anode change addresses both halves of the problem.

Can I just remove the anode and not replace it?

You can, and it is the worst thing you could do. Removing the anode stops the smell immediately because it removes one of the three ingredients. It also removes the only thing protecting the steel tank from corroding, so you trade a smell for a dramatically shortened tank life.

The smell went away by itself. Is that good?

Not necessarily. A smell that fades on its own, particularly on an older unit, can mean the anode has been fully consumed rather than the problem resolving. Check the hot water for rust or discolouration. If it has appeared, the tank is now the thing corroding, and it is worth having looked at.

Does this happen with continuous flow systems?

Very rarely. Continuous flow units heat water on demand and store almost none of it, so there is no warm standing reservoir and no sacrificial anode doing this job. This is a storage tank problem. If you have a continuous flow unit and no hot water at all rather than a smell, start with the six things to check instead.