Your first step should be to make sure the leak is coming from your balcony.
Then you should contact the builder as soon as possible to ask for it to be fixed.
If they refuse, approach Domestic Building Dispute Resolution Victoria which, according to its website, will help owners resolve building disputes. It doesn’t mention apartments specifically, but then it doesn’t exclude them either.
According to its website, it is currently snowed under with applications, which is all the more reason to get in as soon as the builder shows any signs of not wanting to fix the problem.