Check with your own health insurance, they may cover you overseas as well for emergencies, though you may have to pay out of pocket first and then request reimbursement.
I always purchase trip insurance w/emergency medical coverage for any trips outside the US and Canada. AAA's insurance is through Access America, who also sells policies independently via their website.
To the OP: if you are going to go diving in Australia and have any sort of medical condition that you suspect may cause you a problem on the boat's medical clearance form, play it safe and get a Dive Medical from an Australian doctor first. Australia is especially conservative with medical issues, for one thing they will not accept a sign-off or a prescription from a doctor outside the country.