MetService says a weather system sweeping the country is ramping up, as rain and wind begin to lash different areas.
Meteorologist Lewis Ferris told Midday Report wet weather and wind had come in from the north, spreading all the way down to Christchurch.
Flights have been cancelled in Nelson and Christchurch today and MetService has issued more than 20 severe weather alerts across both main islands.
Rain has battered Wellington tonight and surface flooding has been reported through parts of the city.
MetService has also issued a severe thunderstorm this evening for the top of the North Island, down to Auckland and Waikato.
There was also potential for snow to fall in Canterbury, Central Otago and Southland overnight, and there were road snowfall warnings in place across the South Island.
"As we head into tomorrow, cold air will start spreading up the country. That brings snow potential around Canterbury and it's also going to be a run of quite cool days, parts of Central Otago might be fighting to get above freezing over the weekend."
Earlier, Tasman District Council flood duty officer Matt MacLarin said while staff were expecting low rainfall totals, there was a chance of localised downpours.
"The concern I guess will be heavy downpours cause there's sort of the likelihood that there'll be thunderstorms which is what we saw last week in Nelson but with thunderstorms and localised flooding you never know where or when exactly it will hit."
MetService has issued heavy rain warnings for Northland, Auckland, Great Barrier Island, Taranaki, Bay of Plenty, and central regions south of Taupo.
Auckland residents are advised heavy rain is due from 3pm this afternoon.
Mount Taranaki is expected to get up to 220 millimetres of rain before midnight.
The rain will spread across eastern Bay of Plenty and into Tairawhiti from about 8pm.
A strong wind warning is in place for Taihape this afternoon, with Wairarapa and Wellington facing severe gale northwesterlies in exposed places overnight and tomorrow.
Wintry blast on way
The first real wintry blast of the year is due to hit the southern regions later this week, bringing snow to low levels — including Dunedin.
MetService meteorologist Surprise Mhlongo said a low pressure system was expected to move east across central New Zealand tomorrow morning, followed by a cold southwesterly flow, bringing snow to higher levels in parts of Otago and Southland.
A heavy snow watch has been issued for Canterbury, with heavy falls possible above 400m. Accumulations may approach warning criteria, especially about the Mackenzie Country, MetService said in an update on Wednesday morning.
A road snowfall warning has been issued for the Lindis Pass (SH8) between 10am tonight and 3am tomorrow, when up to 5cm is expected to accumulate near the summit.
There also road snow warnings in place for Arthur's and Porters passes, and for the Milford Road (SH94).
There had been a warning in place for the Lewis Pass, but MetService has advised tonight that it now no longer expects snow to settle on the road overnight.
The deep low would then bring a cold southerly flow on Friday morning, which would spread north over the South Island, likely bringing snow to "near sea level" for Canterbury, Otago, Southland and Fiordland, Mr Mhlongo said.
"Dunedin is likely to get some snow too.
"There will be some very cold temperatures.
"We are likely going to see temperatures dropping from Thursday, going into Friday — actually the whole weekend."
While the snow was likely to be significant and may cause disruptions to travel, warning amounts were not expected.
Temperatures in Central Otago were only expected to reach a high of about 3°C-4°C at the weekend.
Farmers were being warned that livestock may be affected by the bitterly cold winds and snow.
"It’s still very uncertain at this point what level the snow will fall to, so I’d just advise that people keep an eye on the forecast updates."
The bitterly cold southerlies were likely to affect much of the country on Saturday, and snow might accumulate to near sea level in Canterbury and Marlborough, Mr Mhlongo said.
However, warning amounts were not expected.
The cold southerly flow was expected to weaken on Sunday.
- additional reporting RNZ
