An elderly woman is dead after being swept out to sea while walking with her partner along the Kohaihai River, in the northwest of the South Island, yesterday.

The 70-year-old was walking along the water's edge in the Kahurangi National Park when they were taken by surprise at about 3.30pm, police say.

"Despite the efforts of the woman's partner, he was unable to prevent her being swept away," Sergeant Paul Watson said.

A search and rescue operation involving two helicopters, several local recreational boats and search personnel was immediately under way.

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The woman's body was recovered from the sea, several kilometres north of where she was swept away about two hours later.

Conditions at the time were fine and clear, with a large swell coming on to shore, Watson said.

The matter has been referred to the Coroner.