Canterbury flooding: Ashburton Mayor Neil Brown urges people not to panic-buy after town cut off by bridge closure

There's been panic in the small town of Tinwald after the State Highway 1 bridge connecting it to Ashburton was closed by severe weather and flooding.

A key South Island transport link, the Ashburton Bridge closed earlier on Tuesday after it started to slump. The closure made travel between Ashburton and Tinwald impossible. 

Tinwald is effectively part of Ashburton - with only the SH1 bridge separating the two.

Speaking from Tinwald on 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.

"One mother we spoke to had kids at home with her husband - she's worked 24 hours at a nursing home and she hasn't slept, and she can't get home," Brownlie said. 

There were also reports Tinwald's lone supermarket - a Supervalue - has seen heavy demand on Tuesday. One woman said her mum had been knocked over trying to enter the supermarket.

"Tinwald Supervalue and Z [service station] are both extremely busy," one resident wrote on an Ashburton community Facebook page. 

"Please be patient and show some kindness to them as they are under extreme pressure at the moment."

Earlier, Ashburton District Mayor Neil Brown urged people not to panic buy.

"Just go to the supermarket and buy what you need and leave some for the other people," he told reporters.

"We will have a route hopefully open by tonight and those supermarket shelves will be stocked when that opens."

It remains unclear when the weather-smashed bridge, a key route for food deliveries to other parts of the South Island, will reopen.

RNZ reported Dunedin and Queenstown supermarkets were already low on stock, with Countdown confirming deliveries in the South Island had been impacted.

"We're working with our suppliers, transport partners and our stores to help minimise any impact to customers but certainly there are some challenges out there today," a spokesperson told RNZ. 

Meanwhile, one woman stranded in Timaru trying to get home to Christchurch from Invercargill is frustrated by the lack of support.

The bridge closure has made it extremely difficult to get from the south to Christchurch. It's 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.

Tracy Phillips told Newshub she's been waiting it out in a Timaru motel.

"We've had no advice and no support, or any way of being able to contact someone to find out where we could get a subsidised accommodation, or maybe food supplies and things like that."

Prime Minister Jacinda Ardern reiterated an alternative route should be open later on Tuesday. She said the bridge would take some time to repair.

"As you can imagine, with the flood levels where they have been - the fact that we have piles that are affected - it's probably going to take some time to even make that assessment," said Ardern, who took to the air on Tuesday to assess the damage.

"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."