There are two excellent spots at the eastern end of the runway. One is an increasingly popular carpark on the eastern access road where, depending on wind direction, you have the ideal spot for takeoffs and landings. It's about 500m away from the runways threshold.
To get there, simply follow George Bolt Memorial Drive (main road from the city side) and turn left at the turnoff for the international/domestic terminals. Go straight, hit the first roundabout and follow it around to the right. At the next roundabout (75m) take the left and your on the main road to Manukau City. Go over the bridge and about 200m up on the left is the carpark. Slightly further on, on the right hand side, is a small clearing that leads to a farms' entrance. Here, you get directly under the planes which is just fantastic!
Hope that's of help. Enjoy your visit to Aotearoa!!