On most cruise lines the minimum age to travel is 12 months with the exception of Royal Caribbean and Carnival Cruise Lines where the minimum age for infants is 6months on Australian based sailings with no more than 3 consecutive days out at sea.
Infants from 6 months can be booked on P&O Australia domestic (coastal cruises) and New Zealand domestic (coastal cruises). On all other P&O itineraries the minimum age to travel is 12 months.