The community support worker was turning into the St Andrew's Church car park in Stanley St when she saw the driver of the truck lose control on the wet road.
The trailer detached from the truck and struck the right front of her car, knocking it 180deg into the footpath.
She then saw the trailer carry on down the road and smash into a 4WD vehicle which was parked outside the Queenstown District Court.
Chris Buckley, of Queenstown, was driving behind Ms Thornton when she was hit.
"The truck there was coming down the hill and the back end started swerving, spun the truck around, the trailer came around and snapped off at the drawbar where it hitches, hit the car, shot downhill, bounced off the kerb and straight down the street, probably doing about 60 [kmh].
"Then it got down to the car outside the courthouse and walloped it hard. It was . . . . . lucky she [Ms Thornton] was all right. There were a couple [walking below the Ballarat St car park] who jumped up on to the wall."
Police are investigating.
