Jetstar offers one way fares from Auckland to Wellington for $35 one way, and from Auckland to Christchurch for $39 one way. This beats the last Jetstar Auckland to Wellington sale by 26% ($45 OW) and the Auckland to Christchurch sale by 44% ($59 OW).
Also on sale is Auckland to Dunedin for $59 and Auckland to Queenstown for $69.