Thunder cells are looming and lightning is flashing off the North Island's west coast as storm-packed weather approaches.
A swathe of the North Island from Auckland south is now under a high alert risk of severe thunderstorms today.
Northern regions are bracing for a day that could see Auckland hit with its worst downpours in years, as upper and central regions prepare for a tumultuous day of lightning and 100km/h wind gusts.
MetService said thunder cells were firing lighting just west of Auckland and southwest of Hamilton. It warned people in affected areas to be on the lookout for threatening conditions and keep safe as the storm passed overhead.
Heavy rain warnings and watches have been issued, stretching from Taranaki across to Coromandel Peninsula.
It includes a high thunderstorm risk to Auckland and central high country this afternoon and evening.
Some mountainous regions have been warned to prepare for a staggering 180mm deluge across the day.
Rain has fallen overnight but it's the coming hours that have authorities on alert, with Aucklanders being warned to prepare for power cuts ahead of predictions the region could see 25mm of rain in an hour - something that hasn't been recorded in three years.
Most of the upper half of the North Island is under the threat of thunderstorms, with Auckland to Taranaki expected to bare the brunt of the volatile weather.
Taranaki has already seen more rain fall in the last 18 hours than all of last November.
MetService says 90mm has fallen at New Plymouth Airport to midday. Last year 79.2mm of rain fell across the whole month.
A high risk of thunderstorms was forecast for the Auckland and central regions right across the day.
It included a warning of hail, 100km/h wind gusts and intense deluges of up to 25mm of rain falling in the space of an hour. There is a low risk of small tornadoes.
MetService is warning strong winds and heavy rain today could cause trees to topple onto powerlines and result in power cuts.
The forecaster said localised downpours of 40mm in a single hour were possible in squally thunderstorms due to hit this afternoon, with a threat of powerful 100km/h gusts.
Those outdoors are being told to watch for flash flooding and slips, and hazardous driving conditions.
There are heavy rain warnings in place for the Bay of Plenty and Taranaki north of Eltham, with between 80 and 100mm of rainfall expected to accumulate in the inland areas from 1am to 7pm on Wednesday.
There are also heavy rain watches are in place for the Coromandel Peninsula, Waikato, the Ruahine and Tararua Range, Kaikoura coast and ranges and Takaka Hill and the Arthur Range.
MetService meteorologist Aidan Pysleman said there would be heavy rain in Auckland today and a risk of thunderstorms for the top part of the North Island.
There was also expected to be up to 25mm of rainfall in one hour, which would be the biggest fall since April 2017.
Vector has put additional response crews and storm management teams on standby, to respond quickly if required.