A slack pressure gradient has convective abilities within, much of the North Island becomes unstable this afternoon, especially inland. The same goes for the lower half of the South Island, once again especially inland.

Northland, Auckland, Waikato & Bay Of Plenty
Humid with a mix of sun and cloud, the odd shower possible. Showers more likely in the afternoon, some could become heavy with thunder especially about the Waikato then easing / clearing overnight. Light winds.
Highs: 22-23

Western North Island (including Central North Island)
Partly cloudy, showers develop in the afternoon especially inland where some may become heavy with thunderstorms then easing overnight. Light winds.
Highs: 19-24

Eastern North Island
Morning sun, isolated showers develop in the afternoon, possibly heavy with thunderstorms then easing at night. Quite humid with light winds tending east to northeast in the afternoon.
Highs: 22-26

Wellington
Partly cloudy, northerly winds.
Highs: 20-23

Marlborough & Nelson
Partly cloudy, north to northwesterly winds.
Highs: 20-26

Canterbury
Sun and some high cloud, isolated showers develop about some inland areas late afternoon then easing at night with a few spits or showers perhaps reaching the coast. Inland South Canterbury could see a heavy shower develop. Winds tending to the south in the morning then east after midday.
Highs: 20-24

West Coast
Partly cloudy, south of Greymouth expect a few showers, maybe even some rain for Fiordland. Light winds.
Highs: 19-23

Southland & Otago
Morning sun possible then cloud thickening and showers develop in the afternoon, some may be heavy (mainly inland), perhaps thunder, easing at night. Light winds tend onshore after midday.
Highs: 20-23