The busiest time for air travel in Aust - NZ is around Easter. In Australia Easter Friday has the most air passengers of any day of the year. All schools and universities are on vacation. For 2017 Easter Friday is 14th April and Easter Monday is 17th April. These are both public holidays.

There is a lot of competition Australia New Zealand on the trans Tasman route. Flights take about 3:15 to 3:45 (east bound is faster due to prevailing winds). Fares are often AU/NZ$225-$350 one way, but can be less or more. Both economy & business fares can be low. Or at peak times can be very high (double or triple the lowest). There is no advantage in buying a return ticket. Usually better to buy 1 one ways, including on different carriers.