Daniel Wark, 37, suffers from post-traumatic stress disorder.

But magistrate Michael Allen refused to discharge him under the Mental Health Act because the assault was too serious and that Wark had abused his position by flashing his police badge during the incident.

Wark was off duty and driving north on the Pacific Highway on December 30 last year when a utility carrying two 17-year-olds pulled in front of him and then cut across a median strip to take a short cut, Mr Allen said.

Wark gave chase, but the teenager refused to pull over until Wark flashed his badge.

Wark, of Jewells, banged on the victim's door, grabbed him by the throat then punched him in the head several times, a police statement said.

He seized the victim's licence, punched him in the head again, walked away, then returned and punched the 17-year-old once more.

The teenager reported the incident at Belmont police station just as Wark arrived with the victim's licence.

Wark told his colleagues that the victim had placed his life in danger.

The victim was later treated for bruising and swelling to his face, mild concussion and soreness around his neck.

Defence barrister Andrew Bright said Wark had been a police officer for four years and had since been diagnosed with post-traumatic stress disorder for which the police force had accepted liability.

"He collapsed into an uncontrollable fury or anger," Mr Bright said.

"He should not be treated as someone who is a criminal, but someone who needs assistance."

Wark is now unemployed.

Mr Allen placed Wark on a one-year good behaviour bond.