A truck which has rolled off the road is slowing southbound traffic on State Highway 2 at Mangatāwhiri.

Police said the driver was taken to hospital with moderate injuries after his truck-and-trailer unit rolled over at about 3.45am today.

The NZ Transport Agency tweeted at 6.30am saying the crash was still partially blocking the southbound lane near Mangatāwhiri Rd.

"Pass the crash scene with care and expect minor delays through the area," it said.

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St John said it received a call from police at 3.51am and responded with two ambulances.

"We treated and transported one patient in a moderate condition to Middlemore Hospital."