Have you considered the Hotel package option with O it is PP but includes transfer to the hotel & to the ship
Then ask for the flights & pay the deviation fee to get the nonstop flights
It may be more expensive in the hotel end but maybe cheaper for the flights & less hassle for your parents
https://www.oceaniacruises.com/australia-cruises/sydney-to-sydney-REG240125/hotels/
Do the math & see what works best
Do not forget the e Visas for both Countries