Louise - I would try the Visa Wizard with them and see if there are any other options - but I think shingle is right. Do they have children in the country and substantial assets? If so they could go the Contributory Parent Visa route. Or they could go the Noncontributory Parent Visa route, but the queue for that is a ridiculous sixteen years at the moment.
Original Nationality: US
Visa #1: PMV (300) through Washington, D.C
Applied: April 2013.
Visa Granted: January 2014.
Visa #2: Subclass 820 (From PMV).
Applied: End of April 2014.
Visa Granted: Early July 2014.
Visa #3: 801 (PR)
Eligibility Date: End of April 2016 (Applied a month prior).
PR Granted: Early April 2017.