You can purchase a policy after leaving your home in Australia, however, there is a 72-hour waiting period before cover for any events begins. In addition, if you purchase while you're travelling, you must purchase cover up to the date you will return to your home in Australia - we don’t offer cover for only the middle portion of your trip.
If you have an existing Everyday Travel Insurance policy that will end whilst you are travelling, and you purchase a new policy before that period of insurance ends, so there is no gap between periods of insurance, then the waiting period will not apply to that new policy.