A Swedish woman swept down a flooded Canterbury river last night survived the terrifying ordeal thanks to an emergency beacon detection system so new it is still in the testing stages.

The woman and her male companion were washed down the Hawdon River, in the Southern Alps east of Arthur's Pass, just after 4.30pm.

The man managed to get to the water's edge of the flooded river flat, but his companion ended up stranded on an island in the middle of the river.

Their emergency locator beacon, set off by the man, saved the woman's life.

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Rescue Co-ordination Centre New Zealand senior search and rescue officer John Ashby, who co-ordinated the rescue, said the new MEOSAR satellite system for detecting beacons meant the pair were rescued an hour later. The woman was severely hypothermic.

"It's still in testing, but it gave us an alert probably an hour earlier than the old system."

That "absolutely" could have been the difference between life and death for the woman, who was plucked from the island by a Garden City Helicopters' crew, Ashby said.

"She was very, very lucky ... it was a good thing we got there fairly quickly."

The weather was not good, but the helicopter crew had agreed to "give it a go", he said.

"It was fairly rough conditions. I believe they had about 50 knots on the nose. He would've been fighting the controls."

The helicopter hovered over the island to pick up the woman, and landed to collected the man.

The couple, who had been tramping and were washed downstream wearing their packs, were taken to Christchurch for medical assessment.

An update on their conditions was not immediately available. Garden City Helicopters also could not be contacted.