This video was posted on the Waikanae Community Info FB page and the story was taken up by Joel Maxwell. We also think it likely that stupid schoolchildren are responsible.

Waikanae River thumbtacks

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.

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.

Full article on the Stuff website