A pedestrian is fighting for his life after being knocked down by a bus on a crossing at a major Sydney CBD intersection.

Police say the elderly man is in a critical condition in St Vincent's Hospital after the accident on the crossing outside the Queen Victoria Building at the junction of George Street and Park Street.

A number 610 Hillsbus service had been turning from York Street behind the QVB into Park and then right into George Street at around 1000 AEST when the accident happened.

Tim Barton, 17 from Sylvania, who sells the Big Issue on the corner outside Woolworths' Metro store at the junction, told smh.com.au: "I saw an old man walking across the road.

" It was a green light (for pedestrians). He was crossing within the white lines. I saw the bus come around the corner.

"I don't think the driver saw the old man walking across the road.

"And then the bus just hit the bloke and the bloke has just bounced off the windscreen about five or six metres."

The windscreen was broken and caved in after the impact.

He said the victim was conscious when he was taken away in an ambulance.

Peter Sundar, 50 from Canley Vale, said: "When he (the pedestrian) began it was green then it started flicking red."

He said the pedestrian light had changed when the elderly man was about halfway across the road. The man had been about three-quarters of the way across when he collided with the bus.

"He was a very short guy, leaning on his left on his walking stick," said Mr Sundar.

Police stopped traffic moving south into Geogre Street from Park Street, but traffic is flowing and pedestrians are continuing to cross at the intersection.

smh.com.au timed the green "walk" sign flashing for six seconds, followed by about 35 seconds for the flashing red before the lights change to red.

Keith Bourke, 68, from Ingleburn, who crossed the junction diagonally - the longest way to cross it - said: "A bloke like me didn't have time to get across the road just then."

After the accident today, most people continued to start crossing the junction when the red light was flashing.

An 81-year-old man who said he was from Paddington but declined to give his name, said: "There's not enough time there. They don't allow enough time at any of the crossings."

As the man crossed the junction diagonally the lights changed to red two seconds before he made it to the other side, having started crossing when the light turned green.

The chairman of the Pedestrian Council of Australia, Harold Scruby, said the phasing of pedestrian traffic lights was designed for athletes rather than elderly and disabled people.

“The pedestrian phase is based on the skills of the likes of Cathy Freeman,” Mr Scruby said.

“If you ... cross legally at the end of the green phase most elderly people haven't even reached half way across the road when it turns red,” he said.

Mr Scruby suggested the Roads and Transport Authority adopt a Danish initiative where large timers were installed at intersections so pedestrians can see how much time is left to cross the road.