Civil Defence has decided against declaring a state of emergency in Southland but is warning schools and businesses to be cautious about re-opening tomorrow because of the risk of more roofs collapsing under heavy snow and rain.

School gymnasiums with flat roofs were at particular risk of collapse, if rain forecast overnight weighed down snow even more, spokeswoman Adrienne Henderson said.

People should check their roofs, particularly on shops where the public would be, and call structural engineers if in doubt before opening, she said.

"We want them to consider the risk to the public before they decide to do that."

Southland has been among the worst-hit from stormy weather nationwide, which has caused road closures and power outages since Friday. Snow flurries have eased but more are expected overnight and tomorrow.

"It is recommended that business owners consider the risks of getting and having staff at work tomorrow and, at the least, delay opening or start times to later in the morning," Civil Defence controller Neil Cruickshank said.

Schools should "err on the side of caution" before opening, he said.

Many Invercargill businesses remained closed today, after several roofs collapsed and others show signs of stress from the heavy snow.

The doors are closed at most businesses including H & J Smith's flagship department store. Some that opened today have had to close early including Harvey Norman, Mitre Ten Mega, Briscoes, E. Hayes and Sons, Waikiwi Countdown and the inner city Countdown supermarkets.

A thick blanket of snow still covers the city and is making driving conditions treacherous.

School closures

Some Invercargill schools have already decided that there will be no classes tomorrow.

Aurora College and Tokanui School will be closed tomorrow for safety reasons.

The Southern Institute of Technology says there will be no classes until midday.

Southland Community College classes in the Menzies Building are cancelled tomorrow.

Concern for buildings' safety

Emergency management teams are assessing several buildings after the roof of Southland Stadium collapsed yesterday, and today it was the turn of Wren's paint shop, in Yarrow Street. Wren's remained cordoned off.

"Luckily the store wasn't open at the time," Mr Cruickshank said.

TV3 reports the Invercargill council has launched an investigation into whether correct procedures were followed when Stadium Southland was built, with mayor Tim Shadbolt saying there have been concerns about its soundness since it was constructed a decade ago.

The roof of the Windsor New World supermarket had also partially collapsed under the heavy snow, while Farmers department store and The Warehouse both had sagging roofs, Civil Defence spokeswoman Adrienne Henderson said.

Chunks of icy snow falling off roofs could cause serious injury, and people are urged to stay clear of building eaves. Southlanders should stay in their homes, travelling only when "absolutely necessary", police said.

People should leave snow on their roof to melt rather than climbing up to clear it.

Road closures

Roads remained particularly dangerous, with ice forming on roads from Southland to Milton.

The Southern Scenic Route between Owaka and Niagara remained closed but the Clinton-Mataura highway and the Milford Road re-opened this afternoon, although chains are required for Milford Road.

Meanwhile, North Island motorists are also being urged to take extreme care when driving in the central region, with many roads affected by the bad weather. Highways in the Waikato, Taranaki and Manawatu regions have been particularly affected by slips and flooding, with some closed or with speed restrictions as crews clean up, the NZ Transport Agency said.

State Highway 41 at Waihi Hill, south of Taupo, between Turangi and Kuratau is closed by a slip. It is expected to re-open tonight.

Power outages

More than 100 Powerco staff are working to restore power to about 2000 households in central North Island regions of the Bay of Plenty, Waikato, Taranaki, Wanganui, Rangitikei and the Wairarapa. Staff were replacing broken poles, clearing trees from lines and repairing damage, Network Operations Manager Phil Marsh said.

Power had been restored to 53,000 customers in the central region from Thames to the Wairarapa since the storm struck on Friday, and Mr Marsh expected the rest to be reconnected by the day's end.

Lambs dying in snowstorm

Southland sheep farmers expect many lambs to die in heavy snow that "couldn't have come at a worse time".

Federated Farmers president and Southland sheep farmer Don Nicolson said in all his life in the area he had only seen one other snow event like this, and that was in the middle of the 1996 winter.

"This couldn't have come at a worse time for farmers. I have never had conditions in the middle of lambing quite like this. Snow isn't normal for Southland as most people think - we don't have penguins running up our main streets."

The best option was to leave the animals to their own devices as interfering could separate lambs from ewes, Mr Nicolson said.

Mr Nicolson guessed that about 300 sheep farmers and 300 dairy farmers would be affected.

Sheep farmer Hugh Collie has more than 4000 ewes on Clinton-Mataura road, near Gore.

"Some of the locals are saying this is the worst we have had at this time of year and we expect lamb deaths," Mr Collie said.

Mr Collie could not say how many lambs had died so far and he would not know until he checked on them in the next few days.

What's in store

The South Island could expect more snow on Tuesday and Wednesday "as our deep Southern Ocean storm sends up one last blast" from the Antarctic, private forecasting service Weatherwatch said.

Head weather analyst Philip Duncan said the cold air was coming from near the Antarctic ice shelf, and would keep temperatures to a high of just 2degC inland or 7-8degC in Invercargill and Dunedin.

Meanwhile, a severe weather warning has been issued for Westland, where heavy rain was forecast across the Westland Ranges on Monday, bringing quickly rising rivers and streams.

Severe weather warnings had also been issued for the North Island regions of Hawke's Bay, Wairarapa, Waikato, Waitomo, Taupo, Taihape, Wanganui, Manawatu and Taumaranui, the MetService said

Westerly gales are forecast to become severe in parts of Hawke's Bay and Wairarapa on Monday, with gusts of up to 130 km/h - strong enough to damage power lines and trees and make driving conditions hazardous.

The levels of the Waikato and Waipa rivers remained high and may stay elevated for an extended period, Environment Waikato emergency management officer Adam Munro said.

"We can expect high flows in the Waikato and Waipa rivers for one to two weeks due to the high level of Lake Taupo, ongoing rain and the already saturated catchments in our region.

"Our flood management system is coping well. However, we will continually reassess rainfall and river flow data and advise the public of any actions they need to take if conditions change."