PR Travel Facility
Your permanent visa grants the right to live in Australia forever. However, the travel facility (ability to travel in and out) expires after 5 years. If you travel overseas after the travel facility expires, you cannot re-enter Australia without a new travel facility. You do NOT lose PR status by being overseas — only the travel facility expires.
Resident Return Visa (155/157)
To renew your travel facility, apply for a Resident Return Visa. Subclass 155 (5 years): granted if you've spent 2+ years of the last 5 years in Australia. Subclass 157 (1 year): granted if you have substantial ties to Australia. Cost: $415. Apply before or after your current travel facility expires — but you cannot travel until approved if it has expired.
Maintaining PR for Citizenship
To be eligible for citizenship, you need: 4 years lawful residence (including 1 year as PR), no more than 12 months total absence in the 4-year period, and no more than 90 days absence in the 12 months before applying. Extended periods overseas can affect both your travel facility renewal and citizenship eligibility. Plan travel carefully if citizenship is your goal.