The line has cancelled all voyages scheduled to depart up to an on 17 September.
Its voluntary halt in operations had previously been due to expire on 31 July.
"The extended pause is being applied as the cruise industry continues to work with relevant government authorities to establish a pathway for the restart of cruising in Australia," said P&O.
Guests with affected bookings will be notified of their options directly or via their travel agent.
President Sture Myrmell said the move was designed to give guests enough notice to plan their holidays with "as much certainty as possible".
