The following sites are useful sources of information about the weather in New Zealand:
The New Zealand MetService - for Forecasts, Weather Maps and Radar Imagery - for licensed or certificated pilots it provides the MetFlight general aviation weather briefing service (username and password required)
NIWA - Weather - New Zealand Climate Update - from the National Climate Centre for Monitoring and Prediction
MetVUW - for the latest New Zealand surface weather observations and radar images
Met.Net.NZ - from Steve Rosie in Christchurch with an excellent set of links
A local enthusiast has established a weather station feeding information onto the web at Miramar near Wellington Airport
Links to other unofficial weather stations can be found at: Current New Zealand Weather
Web cams are also useful for monitoring the weather:
Wellington Harbour Entrance web cams look out from Beacon Hill