State Highway 2 out of Wellington has reopened after a three-car crash caused by an elderly woman driving the wrong way.

The crash, which happened south of Petone early this afternoon, injured five people.

Graham Hewson, who worked close to the scene, said he saw a "little blue corolla'' pull out of no-exit Cornish St on to the highway.

"They would have had to drive past four 'wrong way' signs onto the motorway,'' he said.

The car waited for traffic to pass, then pulled out and drove about 500m the wrong way down the highway.

Hewson heard the impact shortly after.

Police confirmed an 82-year-old woman was driving. She suffered serious injuries, including a broken right femur, and was taken to Hutt Hospital.

The four other people injured had minor injuries.

After the crash traffic was backed up from the Petone offramp to the BP petrol station, almost one kilometre. 

State Highway 2 reopened shortly after 1.30pm. 

- © Fairfax NZ News