caj – in the first instance, the Owners need to be informed that by drilling or otherwise attaching their hotwater heater to the Common Property and consequently by changing the visual appearance of that Common Property, they have breached By-Laws 5 and 17, and the Owners Corporation (O/C) could require them to reinstate its Property by removing theirs and repairing all surface damage.
That’s the hard-line but on the assumption that the Owners are prepared to cooperate, the easier and indeed customary approach would be for the O/C to require the Owners to retrospectively seek its Consent to alter, add, or in this case attach a new structure to the Common Property.
That request should be in writing, and it should be put before a General Meeting in the form of a Motion seeking a Special Resolution to create and register a Special By-Law permitting those Owners and any others who may at some future time wish to install a hotwater heater on the Common Property to do so, with standard conditions such Owners being responsible for the on-going maintenance, repair, and replacement of their property, acceptable installation locations, covering/camouflaging, lagging of hot water pipes (for safety).
The Special Resolution can only pass if ≥75% of those Owners present at the Meeting (personally & by proxy) vote in favour by lot unit entitlement, and ideally any resultant Special By-Law should cover any type of hotwater heater that the O/C is prepared to allow on its Common Property (e.g. storage tanks on the ground, solar on the roof), where, and under what conditions such those mentioned above.
The Owners who have done the wrong thing should be made to pay the O/C’s costs to convene the General Meeting, but as all Owners could conceivably benefit from the Special By-Law, the O/C should cover the preparation and registration costs.
And if they don’t cooperate……post again and I’m sure you’ll receive some practical advice about how your O/C should manage that potentially messy situation.