A serious crash has the Buller Highway down to one lane.

Two people have been flown to Nelson Hospital, one with serious and the other with moderate injuries.

A third person who was initially flown to Buller Hospital with serious injuries, is being picked up by a helicopter from Christchurch and flown to hospital there.

The two-vehicle crash happened around 6.30pm and has closed a lane on State Highway 69, about six kilometres north of the Inangahua Junction in the Upper Buller Gorge.

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Stop/Go traffic management is in place and the transport agency is warning there could be delays.