A woman killed in a head-on crash south of Nelson might still be alive had she been wearing a seatbelt, say police.

Sheryl Ann McFarlane, 57, died after her Mitsubishi RVR collided with a Toyota Camry carrying a family of four American tourists on State Highway 6, near Wakefield about 10.30am yesterday.

One of the tourists, a 54-year-old woman, was yesterday afternoon transferred to Wellington Hospital, where she remains in a critical condition in intensive care with serious head and facial injuries, the Nelson Mail reported.

A 55-year-old man with leg, chest and head injuries and a 20-year-old man with head injuries remain in intensive care in Nelson Hospital. A 23-year-old woman with back and pelvic injuries is in the general ward.

It appeared Ms McFarlane, from Belgrove near Wakefield, had crossed the centre line and collided head-on with the rental car, said Senior Constable Peter Cobeldick of Wakefield police .

Police were unsure what caused her car to cross the centre line.

Mr Cobeldick said it was believed Ms McFarlane was not wearing a seatbelt, something that may have cost her life.