Alright, so here's my theory, and I think it makes sense:
A wizard did it.
No, really. An old Aboriginal shaman lived alone near the rock for about 10 years, practicing magic and shít, and eventually grew pretty lonely and bored. When he saw the girls on their picnic, he was happy to finally find someone to talk to, so he hypnotized them to leave the other girls and to come to live with him. Then, they became his spiritual apprentices.
The reason why the folks didn't find the girls after they were declared missing is because the shaman projected an invisible forcefield around the girls, which made them undetectable.
The shaman taught the girls many interesting tricks, like levitating (which is why Irma didn't have any bruises on her feet when she was later found). Then, Irma somehow snapped out of the magic/hypnotic spell and escaped from the shaman's company without him even noticing it. That's why she couldn't remember anything; she was in a hypnotic trance the entire time.