Finding Rhianna: South Australian police appeal for help solving 23-year child cold case

Adelaide schoolgirl Rhianna Barreau vanished without a trace after buying a card for her pen pal almost 23 years ago.

Police suspect the 12-year-old was murdered and believe someone holds information that could solve her baffling disappearance.

Rhianna was last seen walking away from her Morphett Vale home on the morning of October 7, 1992, during the school holidays.

She went to buy a Christmas card for her pen pal from a shopping centre at Reynella in Adelaide's south, but little else is known about her movements.

When her mother returned home in the afternoon, a Christmas card was there but Rhianna was not and her personal belongings were left behind.

She has not been seen since.

South Australian police have launched a new campaign with Crime Stoppers in a bid to solve a number of the state's cold cases, including Rhianna's disappearance.

A $1 million reward is on offer for anyone who can provide information leading to a conviction in Rhianna's case.