A man has been charged after allegedly stabbing a police officer who responded to a brawl in Sydney's west.

NSW Police were called to Hill End Road in Doonside about 12:30am on Friday, following reports that five men were fighting.

On arrival at the scene, four men allegedly fled in a hatchback, while a fifth man allegedly left on foot.

Officers approached the 20-year-old man who allegedly walked away from the scene.

He allegedly stabbed one officer in the chest with a long knife as police tried to arrest him.

The 34-year-old officer was treated by paramedics before being taken to Westmead Hospital.

He is in a stable condition and will undergo further testing.

The 20-year-old has been charged with wounding a police officer executing duty, wounding person with intent to cause grievous bodily harm, use offensive weapon to prevent lawful detention, custody of knife in public place and assaulting a police officer in execution of duty without actual bodily harm.

Police, with assistance from officers from other local area commands and PolAir, pursued the hatchback.

After a short chase, the car struck a pole and the four men inside the vehicle allegedly ran away.

Police later arrested an 18-year-old man and a 21-year-old man.

The 18-year-old man has since been charged with custody of knife in public place, stalk/intimidate with intent to cause fear of physical harm, armed with intent to commit indictable offence and affray.

Police are still trying to identify two other men.

NSW Police Assistant Commissioner Gavin Wood lambasted the behaviour of the 20-year-old.

"You can't mix words here, it's abhorrent," he said.

"Sometimes you think you've seen it all. But for a young police officer to be out at one in the morning looking after the community, doing his role, and then suddenly he's in hospital with two significant stab wounds — it's beyond belief."

He then further criticised the men involved and implored the public to come forward if they had any information..

However, Assistant Commissioner Wood said the incident "could've been a lot, lot worse".

"There were a number of knives, there were a number of machetes what we will be alleging were in possession of this group," he said.

"And that in itself is a significant concern."

Assistant Commissioner Wood said the injured officer was in "good spirits" and "wanted to go back to work".

The 20-year-old man has been refused bail and is expected to appear before Blacktown Local Court today.

Officers are still searching for the other two men allegedly involved in the fight and subsequent police pursuit.