A person is dead and three injured after a truck overturned in Southland this morning.

Police describe the person who died in the single-vehicle crash, which happened before 2.50am on State Highway 1 between St Ronands and Moore roads, near Edendale, as "very young".

Police say four people were in the truck - the driver and three young people.

St John Ambulance said one victim was in a serious condition and two had moderate injuries. All were taken to Kew Hospital in Invercargill.

No further details have been released as next of kin are still being informed.

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The stretch of highway is closed and diversions are in place.

The Serious Crash Unit is investigating.