kitten – I’m inclined to think that at some stage there would have been one (1) hotwater heater for each Unit in your Plan, and that as those progressively failed somebody (possibly the Strata Manager) has expediently arranged for the hotwater supply to those units to be connected to the single hotwater heater that’s still operational; yours!
However without confirmation by a qualified electrician you still can’t prove to the Mediator and to your Owner/Landlord that the electricity usage of that now remaining hotwater heater is recorded only on the meter that records the supply to your unit.
If you don’t want to pay for an electrician or to sneakily turn OFF the hotwater heater as has been suggested, then contact your electricity retailer (who issues your accounts) and ask them to have the electricity distributor for your area (e.g. Ausgrid or Endeavour Energy) send somebody out to check your meters and connections.
Tell your retailer about the very high usage (kW/h) being recorded by your meters relative to the numbers of residents in your unit, and IF the hotwater is separately shown on your account then give them that usage (kW/h), but don’t confuse the issue by referring to what you think is happening with the supply.
If your issues are well received, then make sure that you’re at home when the distributor’s representative arrives, because that’s when you should tell them what you suspect is happening, and when they’ll check it out and if necessary issue a Notice to your Owner/Landlord to have the wiring properly connected.
In the unlikely event that you get no joy from your retailer, then immediately dispute their account in its entirety and lodge a complaint with the Energy and Water Ombudsman.
That’s far preferable approach in my opinion to you trying to argue logic with the often narrowly focused Mediators at the Department of Fair Trading, although the “fix” will likely be to have the hotwater tank connected to the electricity circuit that services only common property, and that will present another problem with determining with how the individual Owner/Landlords will apportion the Owners Corporation’s electricity accounts for what will then be a communal hotwater service.