A 74-year-old man is in critical condition after he was assaulted while on a routine early morning walk near his home in Greater Western Sydney.

The married father of three was found unconscious on Orchardleigh Street in Yennora about 4.40am on Saturday by members of the public, police said. 

He was treated at the scene for serious head injuries before being taken to Westmead Hospital, where he underwent surgery. His wife and children remain at his bedside. 

'Initial inquiries have led police to believe the man was assaulted while on his regular morning walk,' police said in a statement.

'However, investigators have been unable to speak with the man.'

Acting Fairfield City Police Area Commander Brenton Sommerville addressed the incident during a press conference.

'We're aware the man was performing his usual morning walk, which he does in the area,' he said.

'He had been out for a period of about 30 minutes prior to the incident.'

Neighbour Joshua Albrown claimed he saw a man punch the elderly man.

'The neighbour across the road; one guy, he punched him from the back,' he told ABC News.

'He's a very peaceful man … and I'm still like wondering what's going on with him and why the guy punched him from the back.'

Another local, John Sau, said residents were familiar with the man's daily routine. 

'He's nice and quiet and do walking [exercises] every morning, that's what I saw,' he said.

Det Insp Sommerville told reporters he was grateful for the help from members of the public.

'It happened in the early hours of the morning when an elderly, vulnerable member of the community was left on the roadside, and their actions greatly assisted the police response,' he said.

Officers established a crime scene on Saturday and canvassed the area, during which they collected a number of items for further examination.

Police are appealing for anyone who was in the vicinity of Orchardleigh Street between 4am and 4.40am, and who may have relevant information or dashcam footage, to contact Fairfield Police Station or Crime Stoppers.