A truck has hit a rail overbridge on the North Island's main highway in Horowhenua this morning.

Emergency services were called to the crash, which happened near Otaki around 11.20 am.

The New Zealand Transport Agency said the truck had hit the State Highway 1 rail overbridge near the intersection with County Rd.

Drivers were asked to take extra care approaching the crash.

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A NZME reporter at the scene said the truck appeared to have suffered a mechanical malfunction and struck the bridge before "shearing" along it for a distance.

The truck was left about 100m beyond the bridge and investigators were inspecting it, he said.

A police spokeswoman said the highway closed briefly after the heavy truck and trailor struck the bridge but was reopened by 11.30am.

No other vehicles were involved in the crash and it appeared the driver escaped without injury, she said.