State Highway 94 will be closed for approximately an hour and a half this evening as a crane is brought in to remove an overturned truck and trailer.

The accident happened near The Key, close to the intersection with Lagoon Creek Road, around 1.30pm. The road was blocked in both directions for a period while emergency services secured and inspected the scene. Since then the east-bound lane has remained closed, although traffic has been flowing again with stop-go management in place.

Senior Constable Stu Wright said the crane was expected to arrive about 6pm and the road would be closed while the salvage was carried out. South Roads staff were currently setting up a detour via Blackmount-Redcliff Road, through Weir Road to Hillside-Manapouri Road, to enable traffic to flow.

The NZTA said motorists should plan to allow for an extra hour on their journey time.

The truck and trailer unit were loaded with grain and palm kernel cattle feed, about half of which had spilled onto the road and into the verge, Mr Wright said.

The driver of the truck was shaken but otherwise not seriously injured. He had been checked out by ambulance staff and the Fiordland Medical Centre and since discharged, he said.

It was still unclear exactly what had caused the unit to overturn but it appeared the trailer may have caught on a gravel verge, causing the vehicle to roll as the driver tried to pull it back onto the road.

"How that occurred we don't know. That's the subject of our investigation," Mr Wright said.