Police have called for an outright ban on the sale of petrol-powered, motorised kits for bicycles after the death of a teenage boy in Sydney.

James Ciappara, 14, died when he was hit by two cars whilst riding a motorised mountain bike at Miller, in Sydney's south-west in October last year.

An inquest into the teenager's death was today told the police car had activated its lights and siren in an attempt to stop the boy about 9:15pm (AEDT) that night.

One of the issues the inquest will investigate is why the two officers wanted to stop the boy.

"The bike was not fitted with any reflectors or lights," Counsel Assisting Peggy Dwyer told Glebe Coroners Court.

However, the two officers inside the vehicle said it was not a pursuit.

Constable Jacob Strzelecki said he "turned off the lights and sirens and backed off".

He said they had seen the boy riding erratically.

"He's looked back at us and taken off," he told the court.

"That's when I've seen he wasn't wearing a helmet.

"He looked unsteady on the bike. We needed to stop him for his own safety."

James Ciappara had swapped his standard BMX bike for another boy's motorised bike the day before.

The local community had raised significant concerns about the bikes, which had become increasingly popular, particularly among children.

"It was certainly the speed, it was the noise, it was the way they were being ridden," the officer in charge of the independent police investigation Detective Inspector Darren Newman said about the concerns.

There had been another serious accident involving one of the bikes at nearby Heckenberg weeks earlier.

At the time of the accident, there was a capacity limit on the motors of 200ccs.

Detective Inspector Newman said use of the bikes on roads and footpaths has since been banned but the motorised kits can still be imported and used on enclosed lands.

"When I look at kids riding them, it creates a very dangerous situation in our suburbs," he said.

Detective Inspector Newman said the biggest problem with the motors was their weight relative to the bikes', which makes them difficult to control.

He said they could be even more dangerous in enclosed lands, where there are typically unsealed roads.

The inquest continues.