Just because their insurance doesn’t cover cars doesn’t mean that they’re not liable – it just means they are under-insured. They are probably responsible for damage cause by their negligence (ie, failure to repair the leaks).
I think you may have left your run a bit late … I am pretty sure you won’t be able to to raise a claim under another jurisdiction (every state has its own strata laws, for a start).
But talk to a solicitor in Darwin about their equivalent of the small claims court and make sure you have a reasonable chance of having costs awarded in your favour. Then get them to send a letter to the EC claiming the money (plus the costs of their time). That might be all it takes to get them to pay up.