A collision near Perry Road, Keysborough, has closed all westbound lanes on the Dandenong Bypass.
VicRoads advises that all westbound traffic is being diverted onto Chandler Road to Cheltenham Road.
Motorists can then re-join the Dandenong Bypass from Chapel Road or continue through to Springvale Road.
Delays back to the South Gippsland Highway are building quickly.
VicRoads advises motorists to use Greens Road or Cheltenham Road as alternative routes.
Emergency services are on-site and working to clear the scene.
At 8.20am, VicRoads did not know when the lanes would re-open.
The authority is monitoring traffic in the area and adjusting traffic signals to assist with congestion.
