Puzzled investigators are unsure how a truck driver's body ended up on the other side of the road to his crashed rig.

Hamilton man Jason Steve Cooper died on Tuesday in the Taranaki crash at Mimi on State Highway 3, north of New Plymouth.

The 36-year-old was the only one involved in the single vehicle crash after he failed to take a slight bend and crashed into a dirt bank at about 4am.

However, his body was later hit by an oncoming truck, police said.

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Police said a white station wagon had earlier passed the crash site and flashed their lights at the oncoming truck to warn about the crash.

"As this truck has driven past the crash (on the opposite side of the road), the truck has hit the driver of the crash," police said.

Police now want to speak to the driver of the white station wagon, who may be able to assist them with inquiries.

Police urged the driver to New Plymouth Police on 06 759 5500 or Crimestoppers anonymously on 0800 555 111.