The heavy rain overnight arrived as the trough weather system was set to move northwards across the South Island before weakening over central New Zealand today, MetService said.
A spokesperson for West Coast Emergency Management said evacuations are now mandatory.
"The modelling suggest that low laying areas of Westport are at serious risk of flooding and we must put people's safety first. We put a mandatory evacuation order in place for these at risk areas in Westport to ensure residents safety.
An evacuation centre has been set up at Sergeants Hill Hall, South School Hall, Waimangaro Hall, and Carters Beach Moter Camp.
Residents are urged to follow the evacuation order to keep safe, they said.
Niwa has advised the Buller River is running to 11.4m at Te Kuha.
The modelling shows the situation will be similar to the July 2012 floods.
A spokesperson for Buller District Council said the rain event was under-forecast in terms of intensity and impacts and it has left the district isolated.
The Buller Emergency Operations Centre (EOC) has been stood up again.
"This event was not forecast to have the impacts it has had, but the Emergency Response is in hand, and the Emergency Operations Centre is here to help those who cannot help themselves," said Buller Emergency Management controller Bob Dickson.
"Due to our current isolation, we don't currently have the additional resource present in the district last week."
A state of emergency was declared in Buller late this morning.
The region has endured more record-breaking rainfall, cutting off the town of Westport, forcing people to leave their homes and closing roads.
A week after torrential rain caused flooding in the Buller region, 160mm of overnight rain caught locals by surprise. At least 10 homes have been flooded and multiple homes were evacuated.
People had to leave their homes in Waimangaroa, north of Westport, last night because a slip was threatening their properties. In Westport, firefighters received eight calls from people whose homes had either flooded or were at risk of flooding.
Westport resident Anita O'Brien said today there was a bit more water around this time.
The level of the Buller River is reaching worrying levels and was 11.4 metres high at 12.40pm.
There have already been self-evacuations in some small neighbouring communities, but Buller District mayor Jamie Cleine told RNZ's Morning Report today he won't rule out a repeat of the significant evacuations forced on Westport last July.
In a Facebook post late this morning, Buller Emergency Management recommended that people leave their properties and go to evacuation centres and higher ground, as far as practicable, given the current road closures.
"We encourage you to have a grab bag ready - include medications, water, food, blankets, warm clothes - remember your pets and self-evacuate if you feel unsafe."
Evacuation centres were being set up at Sergeants Hill Hall and South School Hall to welcome people unable to go to friends or families at higher ground.
"Self-evacuation is encouraged for those who experienced flooding in July last year as a precaution. Move your animals to high ground where possible.
If you need to call the EOC please phone 0800 234 533."
The Buller Emergency Management Facebook page, Buller District Council website, and Coast FM are the official channels for up to date information and advice.
MetService meteorologist Lewis Ferris said a record-breaking 160mm of rain was recorded at Westport airport in less than 24 hours.
It "smashed" the record for a 24-hour period set just last week and was also likely to be the wettest month on record, Ferris told RNZ's Morning Report today.
"This has been two very large rainfall events very close together which is very bad news for local residents."
Roads closed
A number of roads are closed because of flooding and slips in the Buller district, including State Highway 6 - which is closed along the coast from Punakaiki to Westport, and inland to Inangahua.
• SH6 Westport to Punakaiki - Surface flooding
• SH6 Inangahua to Westport - Lower Buller Gorge - Surface flooding
• SH6 8 Mile to Inangahua - Surface flooding -
• SH6, SH65 Murchison to Springs Junction - Multiple slips
• SH7 Springs Junction to Reefton - Rahu Saddle - Surface flooding
• SH67 Westport to Mokihinui - Heavy rain
• SH67 Mokihiniu to Karamea - Heavy rain
• SH69 Inangahua to Reefton - Surface flooding
OPEN
• All other State Highways remain open
Meanwhile, Environment Canterbury has issued a flood warning for the North Canterbury alpine rivers.
The Waiau Uwha River is currently flowing at 950 cumecs at Marble Point and was expected to rise further this morning.
"At this stage flows are not expected to get as high as the 1400 cumecs recorded in July 2021, but out of river flooding is possible in some places," a spokesperson said.
- RNZ, NZ Herald and ODT Online
