Hi FromTheNorth,
In Victoria (and could be the same interstate) Section 16 of Water Act 1989 sets out your responsibility for the damage caused by your water leaving your property. However, the aggrieved person has to be able prove it was water coming from your property.
There was interesting case in VCAT. The common property slab had a cored hole, that should have been sealed and as a result that allowed the escaped water to penetrate and damage many more apartments than should have if the hole had been sealed. VCAT then determined that the common property was at fault for some of the claim.
So, whilst at first glance, it is your responsibility, there may be other contributing factors that may be worth investigating. Of course, the legal action you might get drawn into will cost to defend, so have a commercial outlook when thinking about it. It could be cheaper just to make an offer for the repair of her car.