My daughter who lives in an apartment in Melbourne sent me the following:
QUOTE
A question about hot water cylinders... I was reading the little writing on the side of ours, and it says there's a relief valve that should be relieved every 6 months. And it's under a larger heading called 'DANGER'. We haven't done this in all the 5 years we've been here. Then when I went to touch this relief valve it didn't seem like a lever at all. It was all free moving. I'm going to inquire about it when we have our next inspection, coz I think it will be due for it's 5 year check up, but my question is; do you ever release a pressure valve on your hot water cylinder?
UNQUOTE
I would say it's a fair bet the hot water pressure is mains supply and electrically heated.
I hope someone here can help her out, I've copied this link to her. Cheers.

Linkback: https://www.plumbers.nz/q-and-a-hot-water-cylinders/44/danger-sign-on-hwc-whats-it-all-about/1404/