This is not an ad - after some fairly extensive research I have just had confirmed from travelinsurancedirect.com.au that they DO issue cover to expats. their rates are pretty good.
Please see the following e-mail:
HTH, Mr. PThanks for your email.
You are welcome to purchase a policy with us to cover your travels, even if you are already overseas provided you are an Australian Citizen and/or permanent resident and are able to receive long term medical care within Australia in the event of evacuation/repatriation.
Unlike other insurance companies, you do not have to be at home in Australia to take out coverage with us, however please note that the policy will expire once you return to your normal place of residence within Australia.
Kind regards
Travel Insurance Direct