A low pressure system crosses over New Zealand today, northwest winds for the North Island meanwhile a cold south to southwest wind change pushes northwards over the South Island.

Northland, Auckland, Waikato & Bay Of Plenty
Showers, there may be the odd isolated heavy fall / thunderstorm (especially in the afternoon). Sun breaks through at times. Breezy northwesterly winds.
Highs: 17-19

Western North Island (including Central North Island)
Showers, some may be heavy at times with thunderstorms (especially Taranaki). Rain moves in late evening or overnight as north to northwesterly winds change southeast.
Highs: 12-17

Eastern North Island
Mostly sunny with north to northwesterly winds, the odd spit about Wairarapa spreads from the west then becoming more persistent from mid afternoon as southwesterly winds start to freshen. 
Highs: 16-21

Wellington
Rain, more persistent from afternoon as northerlies change southerly.
High: 15

Marlborough & Nelson
Cloudy with scattered rain, becoming heavy in the afternoon for some as southerly winds freshen. Rain eases overnight.
Highs: 13-15

Canterbury
Some rain, easing at night. Snow gradually lowers to 500m, perhaps lowering further to 300m overnight but at that stage it will just be a few flurries. Freshening southwesterly winds, strong at times from afternoon about the coast.
Highs: 8-11

West Coast
Rain eases then clears during the afternoon with some sun breaking through, perhaps not clearing Buller till evening. Southeasterly winds.
Highs: 13-16

Southland & Otago
Morning rain eases to showers, clearing late afternoon / evening. Snow flurries lower to 300m before clearing. Cold south to southwesterly winds.
Highs: 6-10

By Weather Analyst Aaron Wilkinson – WeatherWatch.co.nz