Those rules aren't P&O's decision - they're part of the current Eastern Seaboard Cruise Protocols that all cruise lines need to follow:
"5.1 Masks must be worn during embarking and disembarking, and indoors onboard where social distancing is not possible. Regular reminders will be critical to ensuring passengers adhere to the recommendation.
6.2 Passengers must have a PCR test within 72 hours before boarding, or a self-administered RAT within 24 hours before boarding. Pre-embarkation PCR testing must be done at private laboratories."
https://www.nsw.gov.au/sites/default/files/2022-06/Eastern-Seaboard-Cruise-Protocols.pdf