Permanent residents wishing to travel outside Australia need to obtain a 'resident return' visa every 5 years.  You should not encounter any difficulties if you have lived in Australia for at least 2 out of the preceeding 5 years. However if you have been resident in Australia for a shorter period of time you will need to show close ties with Australia, and if you have been absent for more than 5 consecutive years then you wil also need to show 'compelling reasons' for your absence.

If you become an Australian citizen, you no longer need to worry about Resident Return visas.  To become a citizen, you must have spent a total of 4 years in Australia (lawfully holding a visa at all times), and that in the 12 months before you apply you must have been a permanent resident.  Additionally, in the 4 year period you must not have been absent from Australia for a cumulative total of more than 12 months, and in the 12 months before you apply you must not be absent from Australia for a cumulative total of more than 3 months.  There are very limited exemptions available for this ‘general residence requirement’.