With that length of layover in August (when weather can be wet, chilly and/or changeable), I would spend the time by taking the airbus into the city centre - NZ$22 roundtrip, and a printable discount voucher on their website. Then head a few minutes by local bus or a taxi through a nice coastal neighborhood to the underground Aquarium which is a great way to spend a dry couple of hours, followed by lunch at a great seafood place at the downtown ferry terminal.

If you still have time, a wander around the fish market a few minutes walk or free bus to the west of the restaurant could be interesting (I haven't been for a few years) - otherwise the airbus back to the airport departs from very close to Harbourside restaurant.

You don't mention if both the flights are on the same ticket, but if bags are not able to be checked through there is a left luggage service landside at AKL for a reasonable price.