Why your bathroom smells like rotten eggs

The usual suspect

If your bathroom smells like rotten eggs (sulphur), and the smell is worst when you turn on the hot tap, the cause is almost certainly hydrogen sulphide gas being produced inside your hot water tank.

This happens when sulphate-reducing bacteria in the water react with the magnesium anode rod. The bacteria convert sulphate ions to hydrogen sulphide, and the result is the rotten-egg smell. It is not dangerous in domestic concentrations, but it is genuinely unpleasant.

How to confirm it

Quick test: smell the cold tap separately. If the cold tap smells fine and only the hot tap smells like sulphur, it is the hot water tank. If both taps smell, the problem is in your supply (rare in town water, more common on tank water in rural Hunter properties).

The fix

Two options:

1. Replace the magnesium anode with an aluminium-zinc one. The aluminium-zinc rod does not feed the bacteria the same way. The smell stops within a few days. This is the standard fix and costs $220 to $280 including the rod.
2. Flush and chlorinate the tank. Kills the bacteria without changing the rod. Cheaper short term ($150 to $200) but the bacteria usually come back within a year. I usually only do this as a stopgap.

Will it damage anything?

Not in the short term. Long term, the bacteria can contribute to faster anode degradation, which means your tank loses its corrosion protection sooner. So it is worth fixing not just for the smell but for tank life.

What if it is the drain?

If both taps smell fine and the smell is coming from the drain, you have a different problem. Likely a dry P-trap (the U-bend under a basin or shower that holds water and blocks sewer gas). Run water through any drain you have not used recently and see if the smell stops.

If the P-trap is wet but the smell continues, you may have a venting issue or a broken seal somewhere in the drain line. That is a different conversation.