The Rimutaka Hill Road is down to one lane following a three-car crash near the summit.
The incident happened about 9.30am on the Upper Hutt side of the road, which connects Wellington to the Wairarapa, a police communications spokesman said.
Two people were taken to hospital for precautionary checks, but no serious injuries were reported, he said.
The section of road near the crash site would remain at one lane until the crash was cleared.
The $850m Transmission Gully will be ...Related story: Ball gets rolling on Transmission Gully