A father and son died after a truck and trailer plummeted down a bank near Norsewood early yesterday morning.

Acting area commander inspector Dave White said the 40-year-old man and his seven-year-old son died at the scene of the crash 5km north of Norsewood.

When police arrived just after 12:30am they found the Weatherell Transport truck in dense bush down about a 20m drop.

The truck had gone through a barrier on State Highway 2 before it left the road.

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Due to its location a crane from Palmerston North was called in to retrieve what was left of the wreckage.

Tough weather conditions along with its tricky location meant it was a "mammoth" job for all of the contractors involved.

Tow trucks and an arborist were there early in the day preparing the area for the crane to come and lift the truck and trailer from the crash site, just north of the Manawatu River Bridge.

As traffic became congested and weather conditions worsened the state highway was closed in both directions to allow the contractors to recover the truck and clear the road.

The MacIntosh crane worked throughout the day and a spokesman said they had to be delicate in how they approached the truck with the driver still inside.

The road was re-opened shortly before 5.30pm with a speed limit of 30km in place.

Mr White said motorists were asked to adjust their driving to the conditions as the roads were still wet.

In a statement Mr White thanked the drivers for their patience.

There was more traffic on the road than usual with the long weekend in Wellington so large delays were experienced by drivers.

Mr White extended his sympathies to the families of the two people who died at the crash.

While other people took to Facebook to express their condolences to the families involved.

"R.I.P. Bro an little buddy condolences to the family an friends", "Love and thoughts go out to his partner and 10mnth old daughter xx" and "Love, prayers and hugs to the family. So sad xx" were posted on the Hawke's Bay Today Facebook page yesterday.

The police serious crash unit is investigating the cause of the crash and the deaths will be referred to the Coroner.