Police have charged a man with intentionally causing serious injury after a one-punch assault at Box Hill Hospital on Tuesday night left a surgeon with life-threatening injuries.

The 22-year-old man, from Mill Park in Melbourne’s north, is expected to appear in the Melbourne Magistrates’ Court later Wednesday.

Victoria Police said the victim, aged in his 40s, was leaving the hospital, in Melbourne’s east, when he got into an argument with a man.

Detective Senior Sergeant Ian Shepherd said the man allegedly punched the victim in the head, knocking him unconscious, before fleeing the scene.

He said it was possible the two men had argued about smoking near the hospital, but police had not been able to confirm that.

“The staff member appears to be leaving the hospital. As he’s leaving he becomes in a discussion with an unknown male, the exact nature of that conversation we’re unable to tell,” he said.

“It evolved into a verbal altercation which moved from the outside of the hospital to inside, and it was inside the hospital that the assault took place.”

Eastern Health said the victim was stabilised at the hospital at the time of the incident, before being taken to The Alfred hospital in a critical condition.