The Northern Motorway at Silverdale is heavily congested following an earlier crash.

A van towing a trailer full of firewood crashed just after the Silverdale offramp, and was blocking the righthand southbound lane, a police spokeswoman said.

"Traffic is moving slowly around the crash...but vehicles are having to move into the other lane," she said.

"A tow truck is on the way."

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Police were called to the crash at 11.50am and are still at the scene.

The driver of the van is uninjured and is out of his vehicle.

The NZ Transport Agency tweeted that the crash was blocking the road just after the Silverdale southbound on-ramp, and congestion was building.

The crash has since been cleared but there is heavy congestion in both directions.