At least two northbound lanes on the Southern Motorway, near the Mountain Rd over bridge, are blocked after a truck and at least one car were involved in a crash.

A police spokeswoman said while there was significant damage to a car involved in the crash, an off-duty paramedic on the scene had told police injuries to those involved were minor.

A second car might have been involved in the crash, which occurred about 11.20am, the spokeswoman said.

NZTA said the crash was blocking the two right-hand northbound lanes under the Mountain Rd overpass.

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"Please avoid the area if possible or expect significant delays."

Contractors were on the way to the scene to help with traffic control, she said.

Traffic is gridlocked as long queues form.

Southbound traffic is backed up back to the Harbour Bridge and northbound to Mount Wellington.