You must meet certain requirements to enter Australia, even if you are not required a visa:
The purpose of your trip to Australia must be plausible and comprehensible.
Your passport should be valid for at least 6 months beyond your stay in Australia.
You must be able to finance your own living and travel costs.
You must hold a confirmed return flight to prove that you are prepared to leave Australia before the duration of the permitted stay ends.
You must not pose a threat to security or public order.