The NZ Transport Agency says one southbound lane has now been opened on State Highway 1 at Ellerslie after a truck hit the Penrose overbridge.
The truck’s trailer has flipped and is blocking two southbound lanes.
Police and engineers are at the scene.
Southbound traffic is heavy back to the city and rubberneckers are causing delays northbound back to Princes Street.
Motorists are advised to come off the motorway and consider using Great South Road as an alternative.
Those heading to the airport are advised to avoid the Southern Motorway.