I am feeling very stupid right now.
I should have mentioned that failure of your hot water service comes under urgent repairs and it means that if you can’t contact the landlord or agent (or they refuse to do anything about it) you can go ahead and pay for the repairs yourself ans then demand the money back from your landlord.
With a number of tenants affected, a group of you could split the cost and share the money when you get it back. The landord should be able to claim most of it back from the owners corp anyway.
Have a look at this factsheet on the Tenants Union’s excellent website for more information.