A Swedish couple have survived a terrifying ordeal after they were swept down a flooded South Island river.

Maritime New Zealand posted details of the dramatic incident, and subsequent rescue, on its Facebook page this afternoon.

The Rescue Coordination Centre New Zealand co-ordinated the rescue from a flooded river flat in the Southern Alps.

The couple had set off their emergency beacon after being swept down the flooded Hawdon River, Maritime New Zealand wrote.

"The man managed to get out of the river but the woman was swept 300 metres downstream before washing up on a small island. RCCNZ identified a break in the weather and Garden City Helicopters made the rescue.

"The helicopter crew plucked the severely hypothermic woman from the middle of the river before taking both to Christchurch for medical assessment."

A fast notification from the new MEOSAR beacon detection system and quick action by by the centre and helicopter crew lead to the successful rescue, Maritime New Zealand wrote.

"A reminder for us all: Beacons save lives – get one and take it when you head into the hills."