A rural police officer thought he had seen it all until he drove past a man on a scooter with his underwear pulled down to his ankles.

When stopped, the half-naked motorist explained that he was merely "letting them cool down".

Constable Craig Newman was the lucky officer charged with dealing with the situation on State Highway 1 north of Amberley, Canterbury, about 4.30pm on Monday.

He couldn't believe what he was witnessing. The man on the scooter was wearing a top, but no pants.

"As you came up behind him all you could see was his big white bare bum," Newman said.

"I thought 'that's bizarre'. I didn't think I'd seen it properly."

He pulled the 35-year-old driver of the black and orange scooter over and asked why his pants were down.

"I'm just letting them cool down," was the man's reply.

He had driven half naked for about 10 kilometres after finishing work in Rangiora.

Newman said there had been no complaints about the man's behaviour so he let him go and warned him about the importance of "wearing all safety equipment in the appropriate places".

"I thought I'd seen it all, but I hadn't. I don't want to see any more of it."