Well I'm in Canada and I'm not sure about Australia, but here, if someone is a danger to themselves or others, and won't voluntarily go to hospital, you can call the police. The police will then pick them up and escort them to hospital for compulsory treatment. If he's not harming himself though, here at least you can't do anything, unless he consents. If he does consent, calling his physician would be a good way to start the process, or a local crisis line of some sort.