The power cut has affected people from Nelson and Blenheim to Hanmer and Greymouth and as far south as Dunedin.
Marlborough Lines says at least 26,000 properties were without power this morning in the region. It confirmed about 10am that power was being restored.
"We are gradually relivening the network and expect full restoration by 12pm.
"Please treat all lines as live at all times, especially if any lines have come down — stay well clear and report any hazards immediately."
Many businesses were forced to close due to the outage and some were operating cash only.
Transpower executive general manager, Mark Ryall, earlier told Nine to Noon there were three big transmission circuits for the upper South Island which had all tripped.
Ryall said one circuit had been lost overnight, the second also went down and the third tripped at 7.45am today.
He said two were now working again.
"We're working really hard to stay on top of this but unfortunately transmission lines in severe weather can trip for a number of reasons."
Nelson City Council said the power outage included Nelson, Hanmer, Greymouth and Blenheim.
"Transpower crews are working to restore power. Traffic lights are out across the region, and motorists are being urged to drive with extra care."
Shortly before 10am the council said power was being restored throughout the city.
As power was being restored further north, more than 1600 people in the Hanmer Springs area were without supply around 11am.
MainPower's website says it's aware of the outages and is working to restore supply.
Meanwhile, State Highway 7 from Hanmer Springs turnoff to Springs Junction is closed due to flooding.
Power is also out to thousands of customers in parts of the North Island.
Gusts of more than 200kmh were recorded overnight in exposed areas of the Canterbury High Country already, and more strong winds are expected to hit the wider region and lower North Island today.
A red wind warning is in place in Wellington, Christchurch, wider Canterbury and Marlborough from Cape Campbell southwards.
MetService head of weather news Heather Keats says pretty much all of the eastern side of the country will get strong winds today.
Energy Minister Simon Watts says Transpower has advised him they are still investigating the cause of the fault, but its likely to be the severe winds.
He says they're working to restore supply.
Flights cancelled
At Christchurch Airport at least 55 cancellations are listed on its website, while Air New Zealand has cancelled all flights out of Wellington Airport this morning, after its website earlier showed 67 cancellations.
Air New Zealand is offering flexibility to passengers travelling through Wellington, Christchurch or Dunedin today as severe weather moves across the country.
The airline said travellers booked on flights between 6am and 6pm could change their bookings free of charge to any time within 48 hours of their original flight. Changes can be made directly through the Air NZ app.
Several highways are closed in both main islands, owing to surface flooding, slips and trees coming down.
'Threat to life'
The MetService told Morning Report today's destructive weather would impact all of the South Island and most of the lower North Island.
She said the warnings should not be ignored and there was a threat to life.
"It's set to ramp up from now and only going to get worse."
Keats said the weather system was fast moving so it should improve later today, but more bad weather would return on Monday.
Despite the bad weather, there would be unusual heat, with Hastings reaching 30 degrees Celsius.
"This spring has been relentless," Keats said.
"It does feel like it has been system after system after system."
Keats said the stronger winds this week were more than MetService would usually expect to see in spring.
Raft of warnings
MetService late last night issued warnings for "potentially destructive and damaging winds" across southern regions today.
The forecaster is advising residents to "stay indoors or seek sturdy shelter away from trees".
MetService meteorologist Mmathapelo Makgabutlane said Otago, Southland and Canterbury had already had "one big windy system" this week, but this one was expected to be more intense.
She said the severe gales were caused by a deep low-pressure system to the south of New Zealand, and a high-pressure system to the northeast of New Zealand.
"That difference in pressure between the low-pressure and the high-pressure systems will be really intensifying the winds, especially because they’re in such close proximity, and that low is quite low — possibly lower than 960 hectopascals.
