First of all a cold front swings by early morning, and then we have a rather significant upper cold trough move in early-mid afternoon.
The CVA (Convective Vorticity) from that front is significant, Wouldn't be surprised if an isolated Twister touched down along the coast.
The frontal system moves across the North Island during the morning, bringing a burst of heavy rain to central and western areas. There is a high risk of thunderstorms with this front from Taranaki to Northland, and across to eastern Bay of Plenty. These thunderstorms may produce localised rainfall rates of 10 to 20mm/hr, strong wind gusts of 100 km/h, and hail of 10 to 20mm diameter. A lower risk extends from the central North Island to Wellington, the Marlborough Sounds and Nelson. A trough moves over the west of the South Island early morning, bringing a moderate risk of thunderstorms from Westland to northwest Nelson. These thunderstorms may produce brief heavy rain and small hail.
Also of note, late morning wind convergences are likely to produce showers through inland North Otago and inland parts of the Canterbury Plains up to about Christchurch. There is a moderate risk of a few thunderstorms late morning,producing localised heavy rain and small hail.
No thunderstorms expected over the remainder of New Zealand.