The road between Canberra and Nowra has been closed after a head-on collision that left one woman critically injured and needing to be flown to hospital.

The crash happened about 9.30am on Nerriga Road (also known as Braidwood Road) at Sassafras Hill, about 30 kilometres north of Nerriga, halfway between Braidwood and Nowra.

A woman was critically injured when her vehicle collided head-on with a 4WD. Her vehicle caught fire as a result of the crash.

The woman was flown by helicopter to St George Hospital for treatment.

The driver of the 4WD sustained minor injuries.

The road remains closed with local diversions in place. Police expect the road will be closed for the next few hours as investigations continue.

Anyone with information about this incident should call Crime Stoppers on 1800 333 000 or use the Crime Stoppers online reporting page: https://www1.police.nsw.gov.au/.