The unidentified 52-year-old woman braked on the wet road, but her car still struck the tree, causing its windscreen to break.
Sergeant Steve Watt said it was believed the surprised woman was not injured.
The tree, which was growing on private land, fell on to State Highway 6 at 8.30am.
Sgt Watt said the incident was weather-related after the heavy rainfall overnight and through the morning.
Fulton Hogan contractors were called to clear the road. The delay caused a long line of traffic to Lady's Mile and the Shotover Bridge.