Jetstar is offering return flights to Ballina . Not many Ballina deals posted but seem to be about the same low price as last years Jetstar Birthday sale. From Auckland, limited dates in May only; for Christchurch, scattered dates May to October excluding September. The downside is these fares don't include bags, meals or seat selection. You also have a long layover in Sydney however you can use that to your advantage to check out the city.
Find the cheap dates on Google Flights or use calendar mode on Jetstar. It's a bit cheaper to book 2 one way tickets as it works out cheaper booking from the Australian site for the flight back.