40-year-old cold case remains uncracked

A Sydney teenage girl last seen leaving for school 40 years ago probably ran away and may still be alive, a coronial inquest has found.

Marian Carole Rees was 13 when she disappeared from Hillsdale in southern Sydney in early April 1975.

The teenager often talked of running away from home and had said goodbye to her brother on the morning she disappeared, Magistrate Sharon Freund said in findings handed down on Thursday.

Ms Freund said the disappearance did not appear suspicious and there was no evidence suggesting suicide.

The teenager probably ran away with the intention of permanently cutting ties with family and friends, she said.

"It is likely nothing has changed in the intervening period."

"She may still want to avoid detection and not wish to make contact with her family."

She said she was unconvinced, on the balance of probabilities, that Ms Rees is dead.

Ms Rees would now be 53 years old.

The case has been referred back to the Missing Person's Unit for further investigation.