Police declare 'major crime' after suspicious disappearance of mother Trisha Graf

The disappearance of a mother in South Australia's far north is being treated as suspicious.

Trisha Graf, 41, was last seen in the Roxby Downs area almost a month ago with her Ford Territory found abandoned east of Andamooka on the day she went missing.

South Australia Police have grave fears for her safety. 

READ MORE: Three states to sizzle under worst heatwave since Black Summer bushfires

READ MORE: Home detention for assault-accused MP

Graf had been with friends at the Roxby Downs hotel until at about 12.30am on Friday, December 12.

The pair hit a kangaroo on the way home a short distance from the Andamooka township.

After striking the kangaroo Trisha and her friend continued onto Andamooka where they visited a home in the north-western area of the town.

READ MORE: Aboriginal mother of five named after dying in outback prison cell

READ MORE: Man charged after police vehicle allegedly rammed

At 2am Graf left to travel a short distance home and was last seen alive driving along Dunstan Drive.

Her car was found abandoned near Blue Dam just after midday. 

"This is highly unusual and particularly out of character for her," South Australia Police Detective Superintendent Darren Fielke said.

"The dam has been searched by police divers, polair has been involved in aerial searching and we've also utilised specialised drone equipment," Fielke said.

"Police have spoken to a number of people and in Roxby Downs who have assisted piecing together the movements of Trisha."

Detectives are now urging anyone who saw Graf in the hours before her disappearance on December 12 to contact them.

DOWNLOAD THE 9NEWS APP: Stay across all the latest in breaking news, sport, politics and the weather via our news app and get notifications sent straight to your smartphone. Available on the Apple App Store and Google Play.