State Highway 2 in central Hawke's Bay has reopened after a truck left the road killing two people early this morning.

Police confirmed a 40-year-old man and his 7-year-old son died when a truck left SH2.

A Greaves Bulk Haulage truck and trailer left the road and plummeted down a cliff 5 kilometres north of Norsewood about 12.30am.

The accident site, on the hill just north of the Manawatu River bridge, was congested this morning as tow trucks and an arborist worked to prepare the area for a crane to lift the truck and trailer from the crash site.

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The truck was being contracted by Gisborne transport company, Weatherall Transport.

Police said the road reopened just before 5.30pm. The speed limit is 30km/h and motorists are asked to adjust their driving conditions as roads are wet.

Acting Area Commander Inspector Dave White thanked drivers in the area for their patience and the contractors who had a mammoth job to remove the truck and clear the roads.

The road had more traffic than usual due to the long weekend in Wellington.

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

Other people took to Facebook to express their condolences to the families involved.

Posts like "Love and thoughts go out to his partner and 10mnth old daughter xx", "R.I.P Bro an little buddy condolences to the family an friends", and "Love, prayers and hugs to the family. So sad xx" flooded in to the Hawke's Bay Today Facebook page this afternoon.

The cause of the crash is unknown and the serious crash unit is investigating while police notify next of kin.

The deaths will be referred to the Coroner.