About 10:40am, a 49-year-old Umina man was riding a Harley Davidson motorcycle approximately 2km’s south of the M4/Lighthorse interchange on the M7 motorway when he swerved onto the centre grass median strip to avoid a collision with a truck.

The rider was ejected from his motorcycle and then impacted heavily with a road sign.

Police and NSW Ambulance Paramedics attended the scene and the man was taken to Westmead Hospital where he remains in a serious but stable condition awaiting surgery on a fractured left arm.

Officers from the Metropolitan Crash Investigation Unit also attended scene and one southbound lane was closed while examinations were conducted.

A number of witnesses have been spoken to and police now wish to speak to the driver of a southbound dark or maroon coloured four-wheel drive towing a white-coloured pop-up roof caravan.

It is believed the driver of the four-wheel drive may be able to assist with inquiries and police are appealing for the occupants of the vehicle or anyone who witnessed the collision to come forward.

Anyone with information about the collision is urged to contact the Metropolitan Crash Investigation Unit on (02) 8882 1458 or Crime Stoppers on 1800 333 000.