A former NSW Police senior constable who kicked a handcuffed Indigenous teenager in the face and spat on him has unsuccessfully attempted to have the case dealt with on mental health grounds. 

Christopher Borg, 42, pleaded guilty in January to two counts of common assault over the incident, which took place during a surveillance operation in Western Sydney in September.

But today his lawyer, Warwick Anderson, applied for the matter to be dealt with under mental health legislation, saying the first-time offender's problems pre-dated the incident and he was putting in a "decent effort" to get his mental health back in order.

In a video of the incident played to Sydney's Downing Centre Local Court, the 16-year-old victim sat on the ground near a parked car as Borg walked up to him and kicked him.

The court heard Borg had taken nine months off in late 2020 but found himself in the same position with "difficulties" at work.

"(He) slowly descended into the grips of this mental health problem which resulted in him offending in the way that he did," Mr Anderson told the magistrate.

William Logan, for the Director of Public Prosecutions, said there was significant and overwhelming interest in having such offences dealt with according to law.

"In recent decades there has been enormous public interest, both domestically and globally, in preventing and deterring and punishing police brutality," he said.

Mr Logan said the "compliant" victim was vulnerable not only because he was a First Nations boy, but because he was small in stature and unable to defend himself due to being restrained with his hands behind his back.

"There were a number of duties of care Mr Borg owed to (him)... each of those duties was breached intentionally."

Mr Logan said it was open for the magistrate to find that Borg's motivation was his self-reported "frustration" with how the operation was set up and executed, suggesting the actions were punishment or "police street justice".

He said the spitting was a serious offence because it carried a risk of COVID-19 transmission.

"These acts were not force in connection with a lawful police duty, this was simply an unlawful assault on two occasions against a 16-year-old boy in custody," Mr Logan submitted.

Mr Anderson said his client wasn't aware the boy was Indigenous.

Magistrate Susan Horan accepted Borg had a mental health impairment, namely an adjustment disorder, depression and anxiety.

"I accept there was a pre-existing nature to the diagnoses, however there seems to have been an escalation in relation to this particular incident," she said. 

The magistrate also accepted the teenager was vulnerable due to his age, the handcuffs and the fact that he is an Indigenous person.

She found the kick was not an extension of lawful force being used in the execution of duty, while the spitting — which landed on the teenager's leg — was unacceptable by community standards.

Magistrate Horan declined the application and adjourned the matter so a pre-sentence assessment report can be prepared.

Borg will be sentenced in May.