State Highway 1 remains closed at Rangitata Bridge north of Timaru, following widespread flooding that's left travellers and residents stranded.
The once-in-a-20-year event prompted a state of emergency, forcing evacuation notices for both people and livestock.
"Until the waters recede, we cannot get in to do the repairs needed," said NZTA system manager Pete Connors.
"We have crews and machinery ready to reinstate the highway and ensure it is safe for all road users once the flood waters from the river's tributaries have been contained back into their main flow."
The bridge will remain closed through Sunday, as will several other key links, NZTA says. Upper Rangitata Route 72, over the bridge at Arundel, will also stay closed.
Local hotels and camping grounds have been offering to put displaced people up, as astonishing photos and videos emerge of the aftermath of torrential rain.
The wild weather has caused widespread broadband and mobile outages across the South Island, with 159 cell sites down. Spark said most services were back online around 10:15am.
"Spark would like to remind customers that services are still vulnerable as they are reliant on a damaged cable which could be affected by moving weather conditions," the company said. "Our team will continue to monitor the situation and share further updates as soon as possible."
Timaru Mayor Nigel Bowen is assuring residents they will not be left out in the cold.
"There's a couple of welfare centres open in Temuka and Geraldine... so if people are travelling through, there are some places to go."
He's warning the public not to take any risks and don't go sightseeing.
"Stay away from those highways. You can't get through and you won't be able to get through. Just plan to hang fire and be safe... If you're wanting to view the river, watch the news. Don't go out there. Emergency services have got enough on their hands."
And on the West Coast, a major stretch of State Highway 6 - from Hokitika to Makarora - also remains closed to most vehicles, following heavy rain and strong winds.
There are slips blocking the road south of Hari Hari.
Rising river and lake levels in Otago are being monitored.
Residents in Fox Glacier are being urged to conserve water.
Westland Mayor Bruce Smith is urging communities in the area to check in with neighbours.
"At the present time every settlement in Westland is isolated - individually and from the main road."
But it won't be long until it's business as usual, he says.
"The weather's been strange and we have got some issues out there, but if you look after yourself and look after your neighbours this weather will blow over... The big thing at the moment is for people to do what they've done for the last 100 years - look after each other."
WeatherWatch says while there will be more heavy rain over the west of the South Isnad on Sunday, it's set to be dry on Monday.
Environment Canterbury says the Rangitata River is still at risk of further flooding, but has receded from its previous high.
"While flood flows are now dropping, overflows into those areas already flooding will continue overnight and into tomorrow," it said on its website. "New problems can also develop on the river as the flow recedes and flow channels within the riverbed shift around."