The hot and dry conditions were being driven by the foehn effect, created when a moist air mass moves across a landmass, dumping its moisture but also producing heat, leaving behind warm and dry air.
Western and upper parts of the North Island, which did not see the same effect, would see a relatively mild day with humid northerlies and cloud keeping high temperatures in the low 20s.
Auckland is in for a cloudy day with northerlies and a high of 22C.
Miller said coastal areas in the west would see some heavy clouds and even fog over the next couple of days due to the humid northerly flow.
A severe rain warning was in place for much of Westland over 24 hours from 3pm south of Otira and 9pm about the ranges of Buller and northwest Nelson.
Up to 300mm of rain was forecast, which could include heavy downpours, localised flooding and slips.
"It is going to be a pretty significant event for those areas," Miller said.
There was also a severe wind warning in place from 11pm tonight to 1pm tomorrow for parts of the Canterbury high country, with winds forecast up to 130km/h, and even stronger in exposed places. Wellington could also see some strong winds Thursday morning.
The front would make its way across to the east coast of the South Island tomorrow, spreading rain over the whole island, before becoming slow moving and not reaching the North Island until Friday.
Friday was looking like a wet day for much of the North Island, especially about Wellington and Horowhenua, with slightly less rain for the east, Miller said.
Rain would slowly ease to showers over the South Island on Saturday as the low and frontal system moved away east, with showers spreading over western parts of the North Island.
Today's weather
Whangārei
Often cloudy, chance shower from late morning. Northerlies developing morning. 23C high, 15C overnight.
Auckland
Cloudy periods. Northerlies developing in the afternoon. 22C high, 15C overnight.
Hamilton
Early fog, then fine. Cloud and possible showers from afternoon. Northwesterlies developing afternoon. 23C high, 14C overnight.
Tauranga
Fine morning, but cloud increasing afternoon. Northerlies developing morning. 20C high, 14C overnight.
New Plymouth Cloud increasing in the morning, rain or drizzle from afternoon. Northerlies. 17C high, 14C overnight.
Napier Fine. Warm northwesterlies developing in the morning. 24C high, 17C overnight.
Whanganui Fine spells. Northwesterlies. 23C high, 16C overnight.
Wellington Cloudy. Some light rain developing towards evening. Northerlies strengthening. 17C high, 15C overnight.
Nelson Cloud increasing morning. Occasional drizzle and northerlies from afternoon. 19C high, 15C overnight.
Christchurch Fine. Evening high cloud. Northwest picking up afternoon, evening northeast. 25C high, 14C overnight.
Dunedin Cloud increasing. Few spots of rain this morning and afternoon. Northerlies strengthening. 24C high, 11C overnight.
