ACC has been buying old police radar gear in a covert plan to thwart speeding motorists.

The radars have been mounted on power poles at up to 70 sites nationally, and are intended to fool drivers with detectors in their cars into thinking police are nearby.

Five radars have also been bought by school bus companies to slow motorists around children, and another six have been requested.

Official estimates put the number of motorists with radar detectors at about one in 10.

A Wellington firm, Emergency Vehicle Systems, buys the radars from the police and sells them on renamed as "drones".

They are used as part of a project between ACC's road safety action planning group, police, the Transport Agency and councils.

It has been under way for about a year, and about $7000 has been invested so far.

ACC buys the drones for around $150 and passes them on to the agency, councils and others for distribution.

Injury prevention manager Phil Wright said yesterday that 10 drones were initially trialled in Canterbury for six months before being shifted to the North Island.

While there had been no plan to keep the project secret, ACC had been happy for it to remain under wraps, he said.

"We didn't tell anyone as we thought it would be best if, when a signal went off, motorists didn't assume it was part of this programme," Mr Wright said.

In the last financial year, ACC received claims worth at least $81 million relating to crashes where speed was a factor.

Mr Wright said the radar idea had followed on from a proposal by Harry Duynhoven when he was Labour's Transport Minister to ban detectors.

"[Detectors] are sold to people who are the more hardcore speedsters. They are an essential element in a boy-racer car.

"They are banned in Australia. Even if they were banned here, there is a couple of years' leeway for retailers. We thought, 'What can we do in the meantime?"'

Current minister Steven Joyce's office said yesterday that he was still awaiting advice from officials on the proposed detector ban.

It was revealed in July that police had begun removing radars from their fleet throughout the country.

The drones are also being used by roadwork contractors to attach to their machinery.

ACC refused to discuss the location of the drones.

Pukekohe resident Bob Prangnell discovered three of the devices when his radar detector started going off, despite there being no police cars in sight.

"They're all within 10 miles of my house. I drive those roads reasonably often and something was always giving off a radar when there were no police around ... One day, I slowed down and had a look around and saw these things and took some photos."

Mr Prangnell, who's had a detector for eight years, said the drones were a clever idea.

But he was concerned that if they were placed too close to potentially dangerous areas such as intersections, drivers whose devices beeped might slow down suddenly, resulting in their being rear-ended by other cars.