A "bloody disgusting" attack on swimmers north of Wellington has seen a river booby-trapped with hundreds of thumb tacks.

The tacks were found in a swimming hole and scattered across about 10 square meters of riverbank along the Waikanae River on the Kāpiti Coast.

Waikanae man Dwayne Joy said he was on his way to the swimming hole about 4pm on Monday when he discovered the tacks along the walking path on the rocky banks.

DWAYNE JOY/SUPPLIED Thumb tacks found scattered across the Waikanae riverbank.

The spot is near the State Highway 1 bridge at the southern end of the town.

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"I saw some kids swimming in the first hole, so I thought I'd go around to my preferred hole ... I initially noticed them underneath my feet and then scanned the area, they were pretty much all over the place."

SUPPLIED The swimming hole is near the Waikanae River rail bridge.

The tacks in the water were about a metre or two away from the main jumping spot into the swimming hole.

There were several hundred on the ground, and it took about 10 minutes to pick them all up, Joy said.

He saw about 15 to 20 in the water, but it was too difficult to get them out. It was likely they would get washed downstream into gaps between smaller rocks, Joy said.

DWAYNE JOY/SUPPLIED The thumb tacks discovered on Monday across about 10 square meters of riverbank and in a nearby swimming hole.

He was "90 per cent" certain it was school kids who did it.

"I've got no idea when they would have done it, but they probably would have done it for a bit of a laugh, just being stupid kids."

The behaviour was "bloody disgusting", Joy said on a video he later posted to social media.

JOHN NICHOLSON/STUFF One of the main rivers on the Kāpiti Coast has been booby-trapped with thumb tacks.

Kāpiti Coast District Council parks and recreation manager Alison Law said the tacks had not been reported to staff.

"If we get reports of items in our rivers that pose a risk to public health and safety we will follow up and remove if it's possible, and safe for our team to do so," she said.

Law said responsibility for the river was shared with Greater Wellington Regional Council.

"Our community are our eyes and ears out there and we all have a part to play in helping to keep our rivers clean and safe."

Police said they had not received a complaint about the tacks.