Astonishing footage has captured a bridge collapsing at Franz Josef, with dozens of people stranded in Haast as torrential rain continues to fall on the West Coast.
A state of emergency has been declared in Westland.
It comes as roads have been closed, a main river is swelling and a school has been closed as wild weather on the West Coast rages on.
MetService is now describing the rainfall and flooding as a "significant event even by West Coast standards".
"The Severe Weather Warnings for the South Island rainfall event are still in play," MetService said.
"Monday's forecast of 500mm to 700mm of rain for the Westland ranges is on track.
"This is a significant event even by West Coast standards and there have been road closures from Makarora on the Otago side of the Haast Pass to south of Hokitika and NZTA are advising West Coasters to avoid all but essential travel. "
Pamela Adams, Operations Manager for St John Ambulance, told 1 NEWS around 50 people are stranded in Haast needing accommodation overnight.
She says they are finding places for them at local motels and baches but some may have to stay in the Haast Hall.
Footage of the Waiho Bridge in Franz Josef collapsing was supplied to 1 NEWS showing the dramatic moment it is swept away by the raging river beneath.
Westland Mayor Bruce Smith described the weather as "blowing like you wouldn't believe" as he braved the outdoors this morning.
The Wanganui River, north or Harihari, was "bank to bank" as it pushed up against a bridge, he said. "It's a decent flood folks."
The bank of the Waiho River in Franz Josef was also swelling, running at 8.3 metres high, Mr Smith said.
Motorists have been advised to take care, particularly on State Highway 94 from Te Anau to Milford Sound.
Heavy rain has caused two slips, closing SH6 between Franz Josef and Fox Glacier about 6.50am, the Transport Agency says.
"Our crews are clearing the slips on the Fox Hills, but given the rainfall intensity, we will keep this stretch of highway between the glaciers closed today as we are anticipating more slips and it is not safe for people to be on the highway in these conditions," Transport Agency Network Manager Colin Hey said.
He advised road users to reduce speed, watch out for debris and road crews, and to use headlights.
MetService said that by 7am, 430mm of rain had fallen in Milford Sound and 220mm at Mt Cook, and Niwa warned "prodigious amounts of rain" is forecast today and tomorrow for areas of the western South Island.
South Westland Area School principal Mark Caplen posted on Facebook saying, following advice from Civil Defence and its board of trustees, the school would close today and buses would not be running.
People are asked to keep up to date with school news via Facebook and Skool Loop.
An Emergency Coordination Centre (ECC) at the West Coast Regional Council has been set up with full staff to monitor the large weather system moving throughout the lower West Coast.
Communication networks and power remain fully functional.