The southbound lanes of State Highway One at Ellerslie will be closed tonight for emergency repair work after a large trailer unit hit the Penrose Road overbridge.

The truck clipped the bridge and spilled its contents out onto the motorway.

The Transport Agency said traffic in the southbound lanes is still slow past Penrose Road, as drivers take a look on their way past.

It's telling people to keep their eyes on the road.

The southbound lanes will be closed from midnight until 6am, from the Ellerslie Panmure southbound off-ramp to the Mt Wellington southbound on-ramp.

Motorists are advised to use the alternative route of Great South Road.