It depends on how long you've been overseas. from the AEC:
- if you're just visiting overseas for a short time, and plan to return, you can enrol and vote
- if you are already overseas, you can enrol to vote from overseas
- if you're enrolled, you can register as an overseas voter IF you intend to return to Australia within six years
and the one most appropriate for what you're asking:
- if you are not enrolled, and have been overseas for less than three years, you may still be eligible to enrol if you are:
- an Australian citizen aged 18 years or older,
- intending to return to Australia within six years
Basically by law, unless you have a VALID excuse (as decided by the AEC) you need to enrol to vote, and the above options show you the various ways in which you can enrol. If you're back in the country while living overseas, you should either have enrolled overseas, or in preparation for being in the country during the election, you can use your address while in the country.