The permanent residency visa is simply a travel authority that allows you to fly in and out of Australia for up to 5 years. If you are in Australia when your visa expires, you are still a permanent resident. If you never left Australia, you would never need another visa.
However, if you are overseas when your visa expires, then your permanent residency has ceased.
If you want to continue to travel overseas as a permanent resident after your visa expires, then you will need to apply for a Resident Return Visa (RRV).
To qualify for a 5 year RRV, you must have been physically in Australia for at least 2 years in the 5 years immediately before applying. The time spent in Australia is cumulative, i.e. multiple stays equalling 2 years is acceptable.
If you cannot show a full 2 years of residence, you may be able to apply for a 1 year RRV if you can show that you have 'substantial ties' to Australia. This includes: family, property ownership, employment, friends and business connections etc.
You will also need to show that you have spent some time in Australia since the grant of your permanent residency. Ideally, this would be at least a few months of residence.