A young kayaker who survived 40 minutes in chilly waters off the Kapiti Coast after capsizing can thank a sharp-eyed member of the public, her rescuers say.

The woman was pulled out of the water weak and very cold after someone spotted her in trouble from the land around 9.30am, the skipper of the Kapiti Coastguard rescue boat, Mark Davidson told NZ Newswire on Saturday.

She was seen coming out of the kayak around 200m from the shore in Waikanae and managed to stay with it for a while before being swept away.

"There was a good sea running and the wind had come up so she was very lucky that someone with a good set of eyes saw her in trouble," Mr Davidson said.

A lookout on the Coastguard boat sent to where she was last seen was fortunate to spot her quickly in the choppy sea.

"We threw a rope to her and pulled her out pretty weak and very cold."

She was ferried back to a waiting ambulance where she was treated by paramedics.

Mr Davidson said the woman was wearing a lifejacket but otherwise was wearing light clothing. She was kayaking on her own in the small one-person kayak.

"It's one of those cases where everything turns out happily but it could easily have been a different case," he said.