A woman is seriously injured after a crash involving a train and two vehicles south of Oamaru.

Police received a report of the collision at the State Highway 1 crossing at Reidston, which is close to Maheno about 15km south of Oamaru, at midday, a spokesperson said.

The road is closed and diversions are in place.

The Serious Crash Unit has been advised.

A St John spokesman said one woman had serious injuries and was being taken to Dunedin Hospital by helicopter.

Two other patients were being treated at the scene.

An Otago Daily Times reporter at the scene said a train was stopped across the road at a SH1 crossing.

He could see the two crashed vehicles next to the train, one an SUV on its roof and badly damaged.

The other vehicle was less badly damaged.

A helicopter landed at the site about 12.30pm and left just before 1.15pm.