As Jimmy says, the balcony itself is likely to be common property (unless the building is old, when it kind of might be the owners property – it’s complicated for older strata buildings.) However, if a previous owner renovated the balcony by adding tiles – then there should be a renovation by-law in place, which makes the common property involved in the renovation the responsibility of the owner to maintain and repair in the future. If there’s no by-law about that, then that renovation is an unauthorised renovation, and the Owners Corporation can now negotiate to register such a by-law, or else take action to have the balcony restored to its original condition (by removing all the additions and remediating it to match the original condition of the balcony – which the current owner must pay for). Then, there’s the past repairs, which might have been done by the Owners Corporation or by an owner, and they might be under warranty, or else the party who paid for those repairs are probably now responsible for making any problems arising from those repairs good…
It’s a mess isn’t it? Still the water proofing has failed and water is getting through the balcony floor slab into the Unit below. That slab is clearly common property. But the water proofing membrane (or system – there might not be a membrane, and the tiles could have been the main way of water proofing) is whose responsibility? If damaged by the later renovations or repairs done by an owner, then it’s likely the owner’s job to pay for the repairs. It will possibly work out to be a joint responsibility?
A suggestion is to get an expert to investigate and report on the cause and the solution, and get quotes for the cost of the solution. Both parties can share the cost of getting the report. Or if there is ill-will involved, then each party can get its own report and pay for that themselves. Then they can read the report, and divvy up the cost of the repairs by the proportion of the attributed cause.
Then there’s the 3rd party involved, who should not be neglected – the unfortunate owner who is suffering the consequences of the leaks – they need to ensure that the other parties get a move on and fix things. Certainly, if there are any unnecessary delays, then they need to go to the Tribunal, and get orders to have the problem fixed. The dispute is less about how to fix the problem, but mainly who will pay for that. Concentrate on fixing the problem now, and working out who pays what later.