That depends on the driver license class you hold your P-endorsement for.
Driver licence classes cover cars, motorcycles and heavy vehicles (ie busses), and are based on the type of vehicle and the weight of the vehicle. A P-endorsement allows you to use this particular license class commercially (ie if you want to driver guide in a bus).
Vehicles can be rigid (eg a truck) or a combination (eg a truck and trailer). The class of driver licence required will depend on the gross laden weight (GLW) for rigid vehicles and gross combined weight (GCW) for combination vehicles. Please read the factsheet by the NZ Transport Agency to determine what driver license class you need to have to drive a bus.
Find out some example scenarios here (updated 5/2016).