Two people suffered life-threatening injuries after their car smashed into a tree next to Auckland's Southern Motorway this morning.

The single-car crash happened about 8.20am just before the Panama Rd overbridge in Mt Wellington.

Two people were trapped in their car and had to be freed by the fire service.

Three St John ambulance vehicles were sent to the crash and took two patients to Auckland Hospital, both with critical injuries.

The crash was on the northbound side but was clear of the motorway.

Two lanes have been closed on the northbound side north of Princes St, with a third remaining open.

The Serious Crash Unit will be attending the scene.

At 8.30am the fire service was at the scene but police were still on their way.

A reporter at the scene said a white sedan had crashed into a small tree and was at 90 degrees to the motorway with its rear end sticking out.

Fire and emergency workers were using the jaws of life to cut open the vehicle, she said. The driver's side was completely in the tree and was badly damaged.

Two fire appliances were at the scene and police and ambulance were heading toward the crash, she added.