Yes, a permanent residency visa is only valid for the purpose of entering Australia for 5 years from the date it is granted. You can remain in Australia indefinitely, but if you want to travel outside Australia and to return to Australia more than 5 years after the grant of your visa you will have to apply for what is called a Resident Return visa.
A Resident Return visa usually allows a further 5 years for entering Australia, and should be available so long as you have lived in Australia for 2 years in the last 5.
Of course visas become a non-issue if you are granted Australian citizenship, when you will have an Australian passport.
