UPDATED 4.33PM One of the main routes out of Wellington is closed after a truck crashed.

State Highway Two through the Rimutaka Hills is blocked, cutting off the capital from Wairarapa.

A police spokesman said emergency services were called about 3.45pm.

An eight-tonne truck had rolled. It was the only vehicle involved and nobody was injured.

"The road is closed because of a diesel spill," the spokesman said.

"Were waiting for the Fire Service to get there to clear the diesel."

A Fire Service spokeswoman said one fire truck had been called to the scene.

Transport officials are advising motorists taking the alternative route between Wairarapa and Wellington to add at least three hours to their travel time. 

The only other option for motorists is to take a long detour through Palmerston North and the Manawatu Gorge.