The evacuation boundary around the Rangitata township was extended on Tuesday and residents are still being asked to stay away.
State Highway 1 at the Rangitata Bridge remains closed due to still being submerged in water, following the weekend's wild weather.
The Timaru District Council warns river conditions remain "highly changeable".
"It is not unexpected that the river will continue to scour out and enter other low areas," the council said in a statement.
"We can't afford to become complacent as river levels drop - conditions are extremely dynamic and floodwaters still pose a major risk."
The council says it's still a developing situation. If people feel unsafe or at-risk they should leave immediately.
Motorists were stranded with the South Island being cut in half for three days, as the weekend's flooding closed all available routes between Christchurch and Timaru.
Inland Route 72 reopened on Monday afternoon and remains the only route between the two centres. The council says traffic on the route is heavy but flowing well.
On the West Coast, a logistically challenging exit plan for hundreds of tourists stranded in Franz Josef for days played out on Tuesday afternoon.
Many were stranded for days by the storm-battered State Highway 6, parts of which remain closed.
On Tuesday, more than 400 people travelled in an escorted convoy of more than 150 on the weather-beaten road south to Wanaka.
Local Civil Defence coordinator Wayne Costello told Newshub that motorists would have to drive carefully during the convoy.
"There's a lot of heavily damaged sites," he said.