A truck breakdown has left traffic in gridlock stretching back 12km on Auckland's Southern Motorway.

The New Zealand Transport Agency said the vehicle was blocking the right lane heading into the city just after the Khyber Pass off-ramp.

Just after 10am this morning the vehicle was shifted and all lanes re-opened to traffic.

Motorists were still being warned to expect delays from Otahuhu as the long tail stretched back down the motorway.