A rise in the use of petrol-powered bicycles on roads is putting lives at risk, police say.

Photo: Supplied

Petrol-powered bicycles can reach speeds of over 50 km/h and can become difficult to control, because their brakes and tyres are not equipped for that velocity.

What is considered a fun activity by many young people can quickly turn dangerous, and police are worried it won't be long until someone is seriously injured or killed while riding them.

Head of road policing in Western Bay of Plenty, Senior Sergeant Ian Campion, said he knew of one person injured by the bikes and said they were "becoming a problem".

"For young people, it can be a lot of fun. But they are inherently dangerous if used on a road."

The motors, which are sold in 50, 60 and 70cc sizes, come with a mounting kit and drive system for bicycles.

Suppliers clearly indicated on the product that bicycles fitted with the engines could not be legally used on the road, but some people were even using them for commuting, Mr Campion said.

"Riders using them on the road could find themselves fined hundreds of dollars.

"As a bit of a petrol head myself, I can see the appeal. But riders have to stick to farms and forests and private property."