The ideal time to go to NZ is now onwards ! i.e. mid January ( to avoid the busy school holidays ) to end of February.

The first time I went to NZ was mid November - It rained so much Queenstown was flooded and the west coast road closed due to landslides ( so I then went North where it was warm and sunny ).

Using a backpack as pannier might be possible if you get a light 50L+ climbing style pack with an removable internal malleable frame. Once the frame's removed, you could bundle the contents into two lumps and sling it over the pannier rack horse saddlebag style.