Generally speaking, Personal Effects are goods that have been personally owned and used overseas for a period of 12mths or more proceeding the owner’s arrival in Australia.
However, there are some exceptions, the most common of which is tobacco and alcohol which are duty/GST payable regardless of how long you’ve owned them overseas.
You can declare which goods (if any) are not owned and used overseas for more than 12mths on at section five on your B534 Customs Declaration.