Your Managing (Rental) Agent should have an after-hours contact for emergencies, so I’d suggest that you and the other renters each try that means to advise your respective Agents of the situation, and insist that they or their Clients (your landlord/s) make the necessary arrangements with their building’s Strata Manager.
In the meantime, have a look at this Fact Sheet provided by Tenants NSW, of course bearing in mind that in your situation the hotwater service is communal and therefore the responsibility of the Owners Corporation for your building and not personally the individual Owner / Landlords.
If in any doubt, contact your closest Tenants NSW Office directly by searching HERE – and be thankful that it’s summer where those cold showers are at least tolerable.