Constable Hayden Edwards was a long way from his country home when he was stabbed with a 45-centimetre hunting knife in an unprovoked attack on a busy platform at Sydney's Central Station on Friday night.

In the line of duty and with his back turned, Constable Edwards, 30, was allegedly attacked by a 53-year-old man with a knife as he was speaking to witnesses of an unrelated disturbance at the inner-city station.

Constable Hayden Edwards is recovering in hospital after he was stabbed at Central Station. Credit:NSW Police

The dramatic scene unfolded about 6.45pm on Friday, shortly after Constable Edwards and a fellow officer from the Police Transport Command were called to respond to an assault on Central's platform one.

"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.