Residents are being warned of Rangitata River water levels rising again with Civil Defence urging people to evacuate immediately.
A spokesperson says the south branch of the river is of particular concern.
The Timaru District Council has extended the evaucation area around the Rangitata township. Those living between Rangitata Huts and Lewis Rds are being urged to leave immediately.
"Please do not wait, even if your house has been given clearance by the response team yesterday [Monday]," the Timaru District Council said on Facebook.
"If you feel in immediate danger, call 111."
Council spokesman Stephen Doran said in a statement power is likely to be cut off immediately.
State Highway 1 at Rangitata is still submerged under water and remains closed.
Motorists were stranded with the South Island being cut in half for three days, as the weekend's wild weather closed all available routes between Christchurch and Timaru.
Inland Route 72 reopened on Monday afternoon and remains the only route between the two centres.
On the West Coast, a collection of cut-off communities should be able to get out on Tuesday afternoon.
Part of SH6 remains closed from flood damage, with some stretches of road expected to be out until Christmas.
But NZ Transport Agency (NZTA) spokesperson Pete Connors said temporary road access will be available on Tuesday.
"It's just going to be an opening to get people who want to go south out," he told Newshub.
Connors said the bulk of the roading repairs are still ahead.
"We've got a lot of tidying work to do," he said. "The critical thing is for us to get access."