Hi Susan,
If you'll be living in Port Vila for less then 12 months, then you could look at buying travel insurance which may be suitable to cover emergency type treatment. If you're going to be living there for longer, then expats usually buy local health insurance or an international health insurance policy. Local policies are typically lower in price but may not provide the same level of care that you're used to in Australia.
International policies are open to any nationality and almost any residence location including Vanuatu. You can buy a policy by going direct with the insurance provider, via an insurance agent or via a broker. In all three cases, there shouldn't be any difference in the price you pay.