Generally speaking, the owners corp is responsible for pipes inside common property walls, the lot owner is responsible for pipes inside non-CP walls plus pipes, tapware and fittings outside the walls.
So the chances are that if the cause of the efflorescence is the pipe in the wall then it’s a CP issue and if it’s an external cistern it’s yours. If it’s a built-in cistern inside the common property wall my guess – and it is a guess – is that it’s an owners corp issue.
The OC will therefore also be responsible for replacing the wall and mirror and any other damage done in effecting the repair.