One of the most expensive police cars used in Australia has gone up in flames following a chase in north-eastern New South Wales.

Key points: A police car, worth around $200,000, catches fire after parking in long grass at the side of the road following a pursuit

A police car, worth around $200,000, catches fire after parking in long grass at the side of the road following a pursuit A witness says it was extraordinary how quickly the vehicle went up in flames

A witness says it was extraordinary how quickly the vehicle went up in flames A motoring expert says it was probably due to the diesel particulate filter in burn-off mode

The turbo diesel BMW highway patrol car had been pursuing a sedan being driven by a teenager near Tweed Heads.

The chase ended in long grass on the edge of the Pacific Highway at Chinderah after road spikes were laid, puncturing the fleeing vehicle's tyres.

Murwillumbah resident Geoff Huxley watched the drama unfold after he pulled over when he heard the police siren.

"We were about to drive off and I said to my wife, 'Hang on, I think the police car is on fire'," he said.

A NSW police patrol car engulfed in flames after a highway pursuit. ( Supplied: Marg Huxley )

"I looked over and a fire had developed under the police car and literally within seconds the car was in flames.

"The police tried to put it out with fire extinguishers but it had taken hold.

"It was quite extraordinary how quickly it had taken hold."

Mr Huxley said the other sedan also caught fire.

"I don't know if cars explode these days, but it didn't. It just went up in flames," he said.

"The fire had crept under the car, into the car that was being pursued, and it went up in flames."

Officers attempt to put out the grass fire under the highway patrol car. ( Supplied: Marg Huxley )

Don't park cars in long grass

The BMW-530d was valued around $200,000, taking into account the equipment inside including an in-car video system, number plate-reading technology, front- and rear-facing cameras to capture mobile offences and breathalysers.

Motoring writer Toby Hagan said modern exhaust systems could reach around 500 degrees Celsius which created a significant fire risk.

"Funnily enough these BMW police cars aren't particularly highly powered, but they do have a thing called a diesel particulate filter," he said.

"When that goes into its burn-off mode, it actually builds up even more heat and therefore potentially increases that risk.

"Most modern turbo diesels have a particulate filter and to do it, it builds quite a bit of heat and what you will find is with four-wheel drives, quite regularly, the recommendations are not to park in the grass."

Mr Hagan said this type of police car was not designed by the manufacturers for off-road use.

"They aren't expected to drive over grass too often and are usually confined to bitumen areas, but in this case it has ended up off the side of the road, probably exacerbated by the drought, the dryness of the grass and more prone to catching fire," he said.

"If the police have actually parked on dry grass and it is the exhaust that's caused the issue, clearly there needs to be some training here."

A NSW Police spokesperson said they could not comment on the incident until an investigation had been completed.