I'm not sure if the gas tanks in Australia are any different to American gas tanks, but, **about 1 year ago we had a problem with the pilot light on gas water tank, so we had a plumber come out to fix that. All good, water is hot. But, every single time after someone in the household showers**, about 1-5 minutes after, the pipe used for pressure relieve will unload any remaining water in the tank. Were talking an entire water tank. And it won't just drip out, the water will come out with such force with so much steam and a massive pool of water you'd think a volcano is about to erupt. The force is so powerful that there is a massive hole where the water has been spraying into. (I might have to take a picture of it the next time it happens).
** problem started after pilot light replaced
* turning off the water to the tank causes water to drip out slowly
* our plumber, one that our landlord got, has replaced our pressure valve 7 times - says cannot find a problem
* we have water leaking from the bathroom to the room next door, plumber unsure why - may be part of the problem? Bad plumbing construction from the beginning?

Sounds like thermo-expansion, do you have a expansion tank in the system? If you do the bladder may have failed. Good luck

Nope there isn't an expansion tank, I remember when I turned off the water to the tank, there was a thud on the inside of the tank, like the pressure just filled up all around it. Thanks.

I absolutely agree with Parttime... ;)

Most likely you have a closed system in some way that involves a sprinkler system, a pressure reducing valve, a backflow preventer or some other valve/system that isolates and closes off the water system in a way that won't allow for expansion within the system and the system dumps out...

I'd consider having the landlord install a simple expansion tank in the cold water line to the tank... see image.

I'd also inspect the temperature... make sure you aren't exceeding reqasonable temperatures. This would also cause the issues you are describing and could be dangerous. A reasonable/safe temperature is about 125F... ;)

Mark

Thanks for the responses guys! I'll bring this up with my landlord and the plumber.