A serious crash which blocked both lanes of State Highway 1 north of Wellington is causing massive congestion as emergency services deal with the accident.

A rescue helicopter was also sent to the crash site, which was in the northbound lane, near the intersection with Te Hapua Rd in Te Horo.

A police spokesperson said they received a report of a single-vehicle crash about 10am.

"Two people with serious injuries have been transported to hospital. The northbound lane is closed and traffic is being diverted down Te Hapua Road."

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The NZ Transport Agency is urging motorists to expect "significant delays" and asking motorists to avoid the area or travel altogether.

The rescue helicopter arrived at the scene just before 11am.

Northbound traffic is now being detouring via Te Hapua Rd and southbound is being held at the intersection, NZTA says.