Sydney terror: Police officer stabbed with hunting knife

There were terrifying scenes at Sydney's busiest train station on Friday (local time) after a police officer was stabbed with a large hunting knife.

Constable Hayden Edwards had been called to reports of a fight and was talking to witnesses when the brutal and unprovoked assault occurred.

"We allege that while they were talking to witnesses in that matter, a 53-year-old male approached our constable from behind... with some vigour... and slashed across his back [using] a hunting knife," Police Assistant Commissioner Karen Webb said on Saturday.

A witness said the victim staggered across the platform before collapsing in a pool of his own blood.

"I wasn't sure what was happening. I saw a guy lying face down on the tile area there and I saw a pretty decent amount of blood," witness Jamie Jones told Sunrise.

The accused knifeman then allegedly threatened three other officers before he was tasered and subdued.

Edwards was taken to hospital for emergency surgery on Friday evening and was discharged on Saturday (local time). Police Transport Command Chief Inspector Paul Carrett said he was in good spirits.

"He's obviously still very sore and sorry, very tired, but also very appreciative of the support he's got," he said.

"The stabbing of a police officer, unprovoked, is a very serious matter and something that is concerning to us all the time.

"The bottom line is that policing is a dangerous game and the officer in this has been very lucky and we're very thankful he did not receive a more serious injury."

The 53-year-old accused has been charged with wounding with intent to cause grievous bodily harm, using an offensive weapon to prevent lawful arrest, resisting arrest and custody of a knife in a public place.

He's been denied bail and will be remanded in custody until June 18, when he will appear at Central Local Court.