A New South Wales police officer and volunteer firefighter involved in a car crash in Sydney's south last week has died in hospital.

Timothy Proctor, 29, a probationary constable with the Liverpool Police Area Command, was driving on Heathcote Road at Lucas Heights, in Sydney's south, about 2:00pm on Thursday when another car crossed to the wrong side of the road and collided with his car.

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Mr Proctor was cut out of his car and taken to Liverpool Hospital in a critical condition.

After a three-day battle, he died this morning.

Dash-cam footage captured by the other driver shows the Honda Civic driving southbound in the 100kph zone before drifting to the other side on the road, clipping a Toyota Yaris and then hitting Mr Proctor's red Honda Jazz.

The 65-year-old female driver of the Honda Civic was taken to the same hospital.

She remains in a coma in a critical condition.

NSW Police Commissioner Mick Fuller said Mr Proctor, who decided on a career in policing after quitting his job in IT, was well-respected by his colleagues.

Dash-cam footage shows the Honda Civic hitting Mr Proctor's red Honda Jazz. ( Nine News )

"My thoughts are with Timothy's wife Dianne, his family, friends and colleagues at this incredibly difficult time," he said.

"Timothy was certainly well on the way to a wonderful policing career and was well respected by his colleagues and all who knew him.

"He joined the police force after being a volunteer firefighter and had a tremendous passion and commitment for all emergency services and the community."

Mr Proctor's family requested privacy.

Investigations into the crash are continuing.