Jacinda Ardern shares 'devastating' aerial footage of Canterbury floods

The Prime Minister has shared aerial footage of the 'devastating' damage caused by the large-scale weather event in Canterbury.

The storm caused a state of emergency and hundreds of evacuations as heavy rain and flash flooding swamped the region since Saturday.

MetService issued a rare 'red alert' heavy rain warning on Sunday as persistent rain battered several areas.

Jacinda Ardern on Tuesday shared footage of swarmed paddocks taken from a helicopter as she visited the flood-affected region.

"Devastating to see what communities will be dealing with for some time to come," Ardern wrote on Instagram.

"We'll have a chance to meet with some of the local leaders shortly to see how we can help support them."

The NZ Transport Agency (NZTA) says the Ashburton Bridge on State Highway 1 could reopen to light traffic by the end of Tuesday.

Ashburton was effectively cut in half after the town's bridge was closed earlier due to severe weather and flooding that caused it to slump.

Ardern told reporters on Tuesday work is also underway to get alternative routes set up.

"Work is underway as we speak to try and reinstate some of those alternative routes," she said.

"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."