A truck driver is in a serious condition after a truck and trailer unit rolled in the Coromandel, causing delays to traffic.

Fuel was leaking from the truck, which was carrying molasses, on to State Highway 25 between Tairua and Whitianga, police said.

The road was closed to one lane and "lengthy queues" were forming as no diversions were available, police said.

The crash, on the Tairua side of Whenuakite, would affect traffic travelling to Hahei, Hot Water beach and Whitianga.

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Drivers have been advised that the quickest route to Auckland was now through the Coromandel township and Thames.

The road was expected to be closed for several hours, police said.