Jetstar is offering $29 one way fares from Auckland to Wellington and from Wellington to Christchurch, in any direction for a lot of travel dates. This beats last weeks flight deal by about 17% ($6) when they were offering these flights for $35.
There is also some $25 flights between Christchurch and Wellington for $25 which beats the last sale by $10 (or 29%). These can be found in mid May/June.
Seriously cheap flights.
They are also offering Auckland to Christchurch for $39 which ties with their last sale.