The vision is confronting and disturbing

A woman is pulled over by NSW police highway patrol officers for driving an unregistered vehicle.

But the traffic stop takes a sudden turn when she is arrested for swearing, her head hits the car while she screams that her arm has been dislocated.

Key points: An internal police investigation found the motorist's complaint was "not sustained

An internal police investigation found the motorist's complaint was "not sustained But an LECC investigation found the officer engaged in serious misconduct by using unreasonable force

But an LECC investigation found the officer engaged in serious misconduct by using unreasonable force It was revealed last week LECC only investigated 2 per cent of all complaints due to funding cuts

The incident was among 2,547 complaints assessed last year by the police watchdog the Law Enforcement Complaints Commission (LECC).

But it has been revealed that just 2 per cent of complaints to the LECC were fully investigated due to funding cuts.

The commission recently found two officers engaged in serious misconduct over the racial abuse of two Afghan Muslim women at another traffic stop in Sydney's west.

In a report released last week, LECC chief commissioner Michael Adams QC said it was struggling to process an increasing number of complaints, with cuts of around $6 million over the next four years.

"Like most other public service organisations, we are consistently required to do more with less," Mr Adams said.

"We are, so to speak, drinking from a firehose."

The first traffic stop incident occurred in November 2016 and was investigated by the LECC earlier this year.

In video captured from the patrol car, the woman is clearly upset by her exchange with the police.

"Why don't you go pick up ice dealers and paedophiles?" she tells the officers as they remove her licence plates.

"That's not our job," comes the reply, along with several warnings for her to stop swearing.

As they part ways one officer runs back the woman, grabs her by the arm and arrests her for swearing.

"Are you for real? You just bashed my head against a f***ing window," she screams.

An internal investigation by the local highway patrol found the motorist's complaints of unreasonable force and unnecessary arrest powers were "not sustained".

But the LECC found the arresting officer engaged in serious misconduct and used excessive force against her.

It also found the officer who investigated the incident engaged in serious misconduct by having "such reckless indifference to the outcome that a 'not sustained' finding was inevitable".

Internal investigation could lead to bias

In its findings, the commission said the case highlighted wider issues in the way allegations of misconduct were investigated within the Highway Patrol Command.

The commission noted that it was likely, if not probable, that an investigating officer would have a past or ongoing relationship with the officer who was the subject of the investigation.

The commission was concerned such arrangements could lead to actual or perceived bias in favour of police and suggested changes NSW police could make to improve this.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 4 minutes 15 seconds 4 m 15 s Police camera shows confrontation with motorist

Redfern Legal Centre lawyer Samantha Lee said it was concerning the LECC funding cuts meant it could only investigate 2 per cent of complaints.

"It's a very concerning figure, it means that 98 per cent of cases never see the light of day," she said.

"It sends a very troubling message … that if an officer acts unlawfully then it's highly likely no action will be taken because the watchdog can only watch so many."

NSW shadow minister for police, Lynda Voltz, said funding cuts undermined the public's confidence in the LECC.

"If you've got $6 million coming out you've got less investigations," she said

"There's a huge number of reports [of allegations] coming in. That says that the system is working and that people have confidence to report it. What they want to then have is confidence that they're being investigated."

Premier Gladys Berejiklian said the auditor-general will be looking into the funding issue.