A northwesterly airflow lies over New Zealand today, a cold front moves onto the lower South Island round midday then pushes northwards reaching the lower / western North Island overnight.

Northland, Auckland, Waikato & Bay Of Plenty
Mostly sunny, perhaps some fog to start the day for Waikato. High cloud increases from afternoon then the odd spit later in the day. Northerly winds.
Highs: 16-18

Western North Island (including Central North Island)
Cloud about northern Taranaki and Kapiti may bring the odd light shower, sunny spells elsewhere. Overnight rain pushes into most areas. North to northwesterly winds.
Highs: 14-17Eastern North Island
Sunny and warm with north to northwesterly winds, around dawn on Thusday a southwest change moves into Wairarapa bringing rain. 
Highs: 17-19

Wellington
The odd spit or shower, overnight rain as gusty north to northwesterly winds change southerly.
High: 15

Marlborough & Nelson
Rain about Nelson and the Marlborough Sounds spreads to coastal Marlborough around midday. North to northwesterly winds, changing southerly around dawn on Thursday.
Highs: 16-18

Canterbury
Mostly sunny with developing high cloud, some rain about the high country may be heavy in spots. Late evening / overnight rain becomes widespread as a southwest change moves through. Snow develops on the Alps overnight.
Highs: 17-19

West Coast
Rain with heavy falls, a southwest change pushes through South Westland in the afternoon then North Westland in the evening. Rain eases overnight near the coast but inland about the main divide rain continues with heavy falls possible and snow lowering to 600m.
Highs: 13-15

Southland & Otago
High cloud, late morning a southwest change brings rain into Southland then easing to showers by evening. Rain arrives for Otago mid to late afternoon then eases overnight. Some snow falls on surrounding ranges / mountains.
Highs: 12-17

By Weather Analyst Aaron Wilkinson – WeatherWatch.co.nz