Southland Regional Council says heavy rain has caused two major rivers to rise rapidly and it is warning campers to stay away from river beds.
It says a flood warning is in place for the upper reaches of the Mataura River and, with more rain on the way, Oreti River could also reach the trigger level for a warning.
Duty flood warning officer Wendy Barrow says between 50 and 75 millimetres of rain has fallen in the headwaters of both rivers since midnight on Monday, and up to 50mm more is forecast before 4am on Wednesday.
She says there's concern for anyone camping alongside the rivers in places such as Nokomai and Cattle Flat.
Ms Barrow says as the flood peaks travel downstream, they will also affect the middle and lower reaches of both rivers so farmers should consider whether they need to shift stock.
The Canterbury Regional Council said persistent heavy rain in the Southern Alps is causing the Rangitata River to rise rapidly and people near the river should take extreme care.
Timaru-based flood controller Tony Henderson said that flows in the Rangitata had begun to rise on Tuesday afternoon and were predicted to continue to do so overnight.
He said the Rangitata River usually runs at 100 cubic metres per second (cumec) at this time of year, but by late Tuesday afternoon it was at 180 cumecs and still rising.
Mr Henderson said the council has contacted farmers who are moving stock to higher ground.
The regional council was also monitoring Waitaki River but said it was not causing concern at the current time.