It happened on the B796 Cardoness to Gatehouse Station road near Blackloch, Gatehouse of Fleet at about 1.30 pm last Thursday, April 16.
The collision involved a quad bike towing a trailer and a white Fiat Doblo motor van.
Emergency services attended and a woman was taken to Dumfries and Galloway Royal Infirmary and later transferred to the Queen Elizabeth University Hospital in Glasgow, where she remains.
Inspector Campbell Moffat, of Dumfries and Galloway Road Policing, said: “Enquiries are continuing into this incident and I would ask anyone with information or dash cam footage of the incident, who has not yet come forward, to please get in touch.
If you can help, contact police on 101, quoting incident 1924 of 16 April.