MetService has issued a severe weather watch covering Northland, Waikato, Waitomo, Taumarunui, Bay of Plenty, Rotorua, Taupo, Gisborne, Hawkes Bay, Wanganui, and Taihape until 9pm today.
Auckland has a moderate risk of thunderstorms this afternoon, and showers are likely in the region.
"A combination of humid subtropical air, and afternoon and early evening wind convergences, are likely to trigger heavy showers and thunderstorms about the central North Island and parts of Northland during Wednesday," MetService said.
"These are forecast from Waitomo and southwestern Waikato, across to Taupo, Taihape and the Hawkes Bay Ranges."
There was a moderate risk of these thunderstorms becoming severe, MetService said, producing localised downpours and torrential rain of 25mm to 40mm an hour and possibly more.
"Rainfall of this intensity can cause flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips. Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain," it said.
A severe weather watch has also been issued for Bay of Plenty, Taupo, Hawkes Bay, Nelson Buller, Westland and the Canterbury high country for significant rainfall.
"A broad trough and associated rainband is becoming slow moving over the northern South Island. Steady to moderate heavy rain is likely with this trough until late on Wednesday," the watch said.
"Meanwhile, a moist northeasterly airstream should spread on to the North Island. This is expected to bring some rain to many areas, but about the central North Island, localised convection during Wednesday could bring torrential downpours," it said.
Large swells expected at Gisborne beaches
Good news for surfers: swells reaching 3-4m are expected at East Coast beaches near Gisborne tomorrow.
Mr Glassey said the swells would probably peak around midday.
"Swells that large aren't not common but they're not usual either," he said.
"They'll probably start to ease in the afternoon as the former cyclone moves past."
Humid weather for the next 10 days
Very humid, very warm weather will hang around the North Island, according to WeatherWatch.co.nz.
"The next 10 days have highs hovering around 27C and 30C, which may not sound overly hot to some people but with humidity between 60 and 90 per cent for much of this time the 'feels like' temperature will be around the low to mid 30s," Weather Watch forecaster Philip Duncan said.
"Overnight lows will be tough for sleeping, with temperatures hovering around 20C at the coldest time, and with humidity often closer to 100 per cent at night making it feel like the mid- to late-20s for some."
Winds might provide welcome breaks from the humidity, he said.
Humidity levels have already almost reached 100 per cent in the upper North Island this morning, and temperatures are in the low 20s.
The South Island is also experiencing high humidity, albeit damp and cold. Highs are between 10C and 13C so far this morning.
