The Ashburton Bridge on State Highway 1, a major transport route, has been closed after severe weather and flooding in the area compromised its structure.
Ashburton District Council announced the closure in a statement shortly before 7am on Tuesday, adding that the bridge had been weakened by days of wild weather.
"Ashburton Bridge has been compromised due to severe weather and is now closed," the council said. "Council contractors continue to investigate road conditions and will provide updates when available."
In an update shortly before 9am, officials confirmed the weather-beaten bridge has sustained "significant damage" due to the flooding and will remain out-of-action for at least several days.
Police vehicles have been stationed at the northern and southern ends of the bridge after motorists claimed the structure was "slumping" earlier on Tuesday morning.
Photos obtained by Newshub show the "slump" in the bridge is clearly visible from the roadside, roughly 100m from the northern end. Engineers have been assessing the structure to determine the extent of the damage
It appears the roading has buckled, causing an obvious dip in the surface and the bridge's barrier.
Speaking to Newshub, Ashburton District Mayor Neil Brown said a short-term fix is unlikely.
"It looks pretty major to me - there's one section of the bridge which looks as though it's trying to drop into the river.
"I don't think it'll be days [before repair] - I think it'll be weeks... it's a NZTA bridge so it's them that will do the repair.
"The waterflow has dropped back a huge amount which is good... but I think it could be a day or two for the water to drop further before they actually get into the river to do any work."
The closure of the bridge - a major transport route in Ashburton - has "essentially cut the town in half", says Newshub reporter Kaysha Brownlie, who was at the scene on Tuesday morning.
There is currently no alternative for southbound travellers as all other road options are closed.
"Over the last couple of days, we've been watching torrents of water surging down here, bringing with it massive tree trunks and branches. People have been lining either side of the bridge watching all this unfold, but yesterday they closed the footpaths," Brownlie told The AM Show.
"Today the bridge itself is entirely closed, we understand due to slumping. There is no indication of when it might reopen. It is the only main route through Ashburton."
Prime Minister Jacinda Ardern told reporters at Christchuch Airport on Tuesday afternoon some alternative routes ate expected to be open by the end of the day.
The bridge would take some time to repair, she said.
Torrential downpour had caused the Ashburton River (Hakatere) - the passage of water that runs beneath the bridge - to swell significantly, leading to concerns the water would breach its stopbanks.
Residents were warned to prepare for a possible evacuation as the raging river threatened to overflow - however, the council confirmed on Tuesday morning that the water level had receded considerably overnight.
"Residents can now settle back as there is no longer any need to prepare for evacuation," the council said.
The Waka Kotahi New Zealand Transport Agency (NZTA) confirmed the bridge had been closed following an assessment.
"Due to bridge assessments following prior flooding, the Ashburton River Bridge is now closed," it said.
Motorists have been advised to avoid the area and delay their journeys where possible.
In a post to her social media platforms on Tuesday morning, Prime Minister Jacinda Ardern acknowledged the closure.
"Unfortunately an assessment of the impact of flooding has led to the closure of the Ashburton Bridge on SH1 and there are still a number of other road closures," she said.
"Hoping to get a better idea of some of the wider damage this morning (and what extra assistance is required)."