A large freight truck with hazardous goods on board has rolled in Wairarapa, blocking a highway overnight.

A Fire and Emergency spokesman said the truck crashed on State Highway 2 north of Masterton just after midnight, blocking the highway.

The driver escaped without injuries but the truck's cargo included dangerous material. The spokesman said a specialist hazardous substances unit was at the wreck.

He did not know what the threatening product was.

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Up to six fire crews and police were still at the crash.

One lane on SH2 at Mikimiki Rd was cleared early this morning but traffic control is still in place.