Informal:
* See ya: This is the most common and versatile.
* Cheers: Often used as a general farewell, meaning "thank you" or "goodbye."
* Catch ya later: Implies you'll see the person again soon.
* Later: A shorter, more casual version of "catch ya later."
* G'day: A greeting, but also used as a farewell, especially in a more rural setting.
Slightly more formal:
* Ta-ta: More playful and a bit old-fashioned.
* Ciao: Borrowed from Italian, but widely used in Australia.
More specific:
* See you around: Suggests you'll likely encounter the person again, but not necessarily soon.
* Safe trip: Used when someone is leaving on a journey.
The best option depends on the situation and your relationship with the person you're saying goodbye to.