Alternative routes to the weather-smashed Ashburton Bridge are expected to be open by the end of the day, Prime Minister Jacinda Ardern says.
Ashburton was effectively cut in half after the town's bridge was closed earlier on Tuesday, due to severe weather and flooding that caused it to slump.
The bridge sits on State Highway 1 and the closure meant it was impossible to travel south, with alternative routes already knocked out by flooding - a major blow to the South Island's transport network.
"The Ashburton Bridge on SH1 (the bridge that links Ashburton to Tinwald) remains closed for the foreseeable future.
"Please do not queue at the bridge, as access is not possible at this time," the Ashburton District Council said in a statement.
Ardern told reporters at Christchurch Airport on Tuesday the bridge would take some time to repair.
"Work is underway as we speak to try and reinstate some of those alternative routes. At the moment, we're being advised that those should be reinstated by the end of today but extensive work does need to happen in order for that to be successful."
Speaking from the south side of the bridge earlier, Newshub's Kaysha Brownlie said some residents crossed the structure before its closure on Tuesday morning and are now stuck on the north side.
The closure also made it extremely difficult to get from the south to Christchurch. It was possible to get from Timaru to Christchurch via the West Coast but would take more than 13 hours to complete - compared to a usual 2.5 hour journey down SH1 via Ashburton.
Usually, commuters would be able to divert from SH1, via the Upper Rangitata Bridge at Arundel on Route 72, before moving on to Christchurch - but this route has also been closed by flooding.
"Route 72 is not State Highway so this closure will not appear on the NZTA journeys website," the Timaru District Council said.
"There is currently NO ROUTE NORTH to Christchurch via this route or SH1.
"Roads in this area are in a dangerous condition. Please do not try to drive in this area.
"There is no route north from Timaru to Christchurch at this time, nor is there an indication when this will be reestablished," the council warned.
Earlier, Ardern flew south to walk around Ashburton and assess the enormous damage. She said reinstating roads would be a high priority.
"It is quite devastating in some areas," she said. "There's a lot for us to do alongside farmers to support them in their recovery."
More cash is also on the way for flood-affected farmers in the region following the declaration of an "adverse event".
Agriculture Minister Damien O'Connor said the flooding put further pressure on farmers who were already struggling with drought conditions in the region.
"We will continue to assess whether further support is required as the full extent of the flooding becomes clearer."