There was nearly 20km of heavy traffic on the M2 this morning.

The Transport Management Centre has attributed the 20-minute closure of the Sydney Harbour Tunnel to an overheight truck.

NSW Police had told Ray Hadley the closure was in fact due to a malfunction in the height warning system.

But Transport for NSW Executive Director of Customer Journey Management Craig Moran has confirmed the truck will be fined for being overheight and disobeying warning signs.

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Image: Getty