Cyclist killed in M4 truck smash: police

A cyclist has been killed and three others are lucky to be alive after a truck ploughed into the group in a "horrific" accident on a Sydney motorway, police say.

The cyclists, including two siblings in their 20s, were struck while pedalling along the M4 at South Penrith early on Saturday morning.

They were not long into their trip from Sydney's western suburbs to the Blue Mountains when the B-double truck allegedly veered into the emergency lane on the 110km/h stretch of road.

The eldest of the group, a 35-year-old from Wollongong named as David Williams by the Seven Network, was dragged along by the truck which came to a halt 900m after impact, just after The Northern Road overpass.

Inspector Rick Cox told AAP the man died instantly and sustained "horrendous injuries," adding to the trauma of the incident.

"It appears he was dragged a distance along the roadway, as you can imagine with such a large vehicle," he said.

The others were lucky to come out alive.

A 25-year-old man of Edensor Park suffered a fractured vertebrae and internal injuries, while his sister, 20, sustained head injuries and an arm fracture.

A third man, aged 23 from Bossley Park in Sydney's west, also fractured his arm.

"When you consider the size of the B-double truck and the pushbikes, they were very lucky to get away they way they did," Inspector Cox said.

"There was lot of debris.

"Very horrific, probably one of the worst I have seen in my service.

Their trip was a training exercise ahead of next month's Sydney to Surfers charity bike ride, which raises money for Youth Off the Streets, Seven reported.

The survivors are now in a stable condition in Nepean Hospital, while the truck driver, a 27-year-old Glenmore Park man employed by South Penrith Sand and Soil, was ordered to undergo mandatory blood and urine tests.

He was also treated for shock at the scene, Inspector Cox said.

A motorist who saw the accident rang triple-zero, but crash investigators are still trying to piece together the moments before the 7.40am (AEST) smash and are appealing for witnesses to come forward.

Inspector Cox said it is not known how long he had been behind the wheel.

WorkCover NSW confirmed it was investigating whether fatigue was a factor.

Anyone with information should contact Crime Stoppers on 1800 333 000.