Woman's legs crushed in truck roll back

Perry Duffin and Julia Carlisle

Police have launched an investigation after a semi-trailer rolled backwards into a female motorcyclist, crushing her legs, on a busy Sydney road during the morning peak-hour commute.

It's believed the woman was stationary on Spit Road in Mosman when the truck rolled into her about 7.45am on Monday, a NSW Ambulance spokesman said.

Three units treated the woman at the scene before rushing her to hospital with significant injuries to her lower legs.

Paramedics said it was possible she could lose both her legs.

The woman remains in Royal North Shore in a stable condition, a hospital spokesman told AAP, but they couldn't reveal the extent of her injuries.

Traffic was extensively backed up in both directions on Monday morning as the police crash unit pieced together exactly what happened.