A Carterton man is fighting for his life after being involved in a hit and run.

Police believe the 42-year-old was hit by a car on State Highway 2 near the intersection of Plimsoll St sometime between 1am and 2am today.

The driver fled the scene.

Detective Senior Sergeant Barry Bysouth said the man was now in a critical condition in Wellington Hospital.

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The injured man was discovered by a member of the public about 2am, he said.

"We believe this incident happened between 1am and 2am today.

"We are appealing to the driver of the vehicle to come forward."

He also urged any witnesses travelling on that stretch of road earlier today or anyone who might have information to help them identify what has happened to come forward.

Police would be closing SH2 from 12.30pm today for about 45 minutes to conduct a scene examination.

Diversions will be put in place between Park Rd and Somerset Rd while this occurs.

Anyone with information is asked to contact Police via 105 or anonymously via Crimestoppers on 0800 555 111.