How old is the building? If it’s less that six years old you might be able to get it fixed by the builder/developer as a defect.
Otherwise, it’s a question of whether or not this is a common property wall. The Fair Trading Common Property Memorandum says the pipes within a common property wall are the responsibility of the owners corporation but pipes downstream from the stopcock, that service only that lot and are in a lot property (not common property) wall are the lot owner’s responsibility.
What’s a common property wall? External walls separating your unit from another unit or from common property (e.g. hallways or the exterior of the building). I’m not sure if an internal supporting wall that’s part of the building’s structure is common property or not. But if the wall concerned is external then the piping is the owners corp responsibility even if, as they say, the taps are yours.
By the way, the Common Property memorandum is supposed to have been adopted as a by-law for it to be truly effective but you can bet Tribunals will use it as a guide if it hasn’t.