The girls' disappearance is one of the nation's most notorious unsolved crimes, but Joanne's sister, Suzie Ratcliffe, says there is now renewed hope for a breakthrough in the case after a social networking appeal for clues delivered the new tip.

"While people are talking about the case, there is always hope," Ms Ratcliffe said.

Nearly four decades ago, this is what Joanne Ratcliffe, then 11, and Kirste Gordon, then 4, looked like about the time they disappeared from the Adelaide Oval.

She contacted SA Police's major crime branch two weeks ago with new information from a South Australian woman prompted by a Facebook page highlighting the missing persons case.

The nature of the information is unknown but police yesterday confirmed they were assessing the report. It is the first time Ms Ratcliffe, who was born a year after Joanne's disappearance, has been directly contacted with information about the case.

It comes as a team of private investigators revealed they had gathered a cache of items over the past three years which they believe are linked to the crime.

Joanne, 11, and Kirste, 4, were abducted from Adelaide Oval during a football match on August 25, 1973.

"It's time Joanne came home," Ms Ratcliffe said.

"When I was first contacted two weeks ago, it scared me a lot. If something comes of it, or of the work others have done to try to solve this case, it would give us somewhere to physically grieve.

"We are over wanting to know who did it, how they did it and why  they did it.

"We want to be able to lay (Joanne) to rest and lay my dad's soul to rest also."

Ms Ratcliffe said her contact with SA police had been minimal in the past five years and her mother, Kath, said neither police or the team of private investigators had informed her of any progress on the case.

"I have respect for the Major Crime detectives  ...  they work hard and work long hours and I know they have a lot of cases to work on," Ms  Ratcliffe said.

"The case has changed hands many times and whenever a new detective has handed it, it does bring  a fresh look at it." Police said that any new information relating to the case would be assessed and followed up - but all leads had so far  been discounted.

"There have been a number of reviews in relation to this case and every piece of information has been looked at and discounted thus far," a police spokeswoman said.

The private investigators, who wish to remain anonymous, believe credible information they have gathered has been ignored by police.

They have spent almost three years searching for clues to the case since being leaked confidential documents about the crime.

Major Crime detectives told The Advertiser they were satisfied there was "nothing in" evidence produced by the private investigators.

Key pieces of the puzzle the private investigators believe they have gathered, from the Mid North area of Yatina, include a hat which matches one worn by the suspected abductor, a medical book and newspaper clippings detailing the crime taken from a country house named as the "crime scene" in the confidential documents.

They also found, in a sealed and submerged tunnel in an area mapped out in the documents as the burial site of the girls, two barrels containing a honeycomb-like substance which has tested positive for blood.

A presumptive test of the material for blood, conducted by Police Forensic Services, found a "weak to very weak" trace of blood, police say.

A secondary test for haemoglobins ruled the barrels as insignificant to  the case.

The private investigators have stored the barrels and had other tests on the material conducted which detected traces of blood and acid, but it is unclear if the blood is human.

The confidential documents which sparked their investigation were produced in 2007 and leaked from State Archive files of the Mullighan Inquiry into abuse of children in care.

Police have previously dismissed the documents produced by a convicted pedophile while serving a Yatala jail sentence as a fabrication from the writer's imagination.

Those documents claim the person who produced them witnessed the murder and burial of the girls and has information on other unsolved crimes.

Source: news.com.au