Thunderstorms are gathering over Mt Taranaki and the central and lower North Island, and swathes of the country are under severe weather watches.
In storm-lashed Roxburgh, Mayor Tim Cadogan and the police are warning people to be ready to evacuate if more wet weather causes further flooding.
MetService has severethunderstorm watches in place from Waikato to Wairarapa in the North island and from Marlborough to Dunedin in the South Island.
The weather forecaster is expecting thunderstorms and heavy showers to kick in this afternoon, carrying on until the evening.
For southern Waikato, Waitomo, Taumarunui, Taupo, Taihape, inland parts of Taranaki, Whanganui, Manawatu and the ranges of Hawke's Bay and Wairarapa some of the thunderstorms could become severe, producing localised downpours of 25 to 40mm an hour, MetService said.
The same warning was in place for the South Island in the ranges of Nelson and Marlborough, the Canterbury High Country, Central Otago and inland parts of North Otago and Dunedin.
Rainfall that intense could cause flash flooding, slips and hazardous driving conditions, MetService said.
Today's forecast for 3pm. Areas in red are experiencing thunderstorms. Photo / The Weather Company
WeatherWatch head analyst Philip Duncan said a shifting number of downpours could be expected across the country over the next 10 days.
More thunderstorms were possible tomorrow and wet weather should persist until easing on Sunday.
"It seems the biggest downpours will slide a little further north up both islands over the coming few days, which will allow heat to return to the lower South Island under sunnier skies," Duncan said.
Highs should be back up to 30C later this week.
"The northern shift will put far more people in the risk zones for downpours, especially as they form in or drift into regions like Waikato, Bay of Plenty and Auckland."
Heavy downpours with thunderstorms were now forming around Taranaki, he said.
The majority of thunderstorms were around Central Plateau and State Highway One and the Desert Road.
Duncan advised motorists take extreme care in very heavy downpours which may cause surface and flash flooding, and hail.
Thunderstorms were also growing and forming along the Ruahine and Tararua Ranges.
Storms will continue to grow until mid to late afternoon in the North Island while in the South Island thunderstorms may form in the northern mountains this afternoon, while downpours could flare up until this evening elsewhere, as far south as Central Otago.
"Here we go again," a WeatherWatch tweet said, urging people to take care as the storms wouldn't peak until mid-late afternoon.
Here we go again! #Thunderstorms now over Central Plateau & forming over Mt Taranaki. Take care, storms don't peak until mid-late Afternoon. Maps as of now: pic.twitter.com/KWClruzpDg
Heavy downpours caught Roxburgh locals by surprise on Sunday afternoon as a MetService weather station recorded almost 30mm of rain in one hour and a temperature plummet of 5C.
The flash flood sparked evacuations and cut off power and water. Residents have been told to expect to be without water until at least tomorrow.
Evacuations in Roxburgh on Sunday afternoon were short lived but the town remained cut off from the South because of slips on State Highway 8 and Teviot Rd.
The Central Otago District Council issued a boil water notice for the area after a water pipe blew out. They were also asking residents in the area to conserve water overnight.
