The mother of a seven-year-old girl believed to have been murdered more than 40 years ago has been ordered to pay legal costs to a child killer suspected of her daughter's murder.

Linda Stilwell disappeared from the St Kilda foreshore in 1968 and her body has never been found.

A coroner has found convicted child killer Derek Percy was on the beach at the time of Linda's disappearance.

Linda's mother, 72-year-old Jean Priest, lost a Supreme Court bid to have Percy testify about Linda's death on the grounds his evidence would be unreliable.

The court ordered Ms Priest to pay Percy's legal fees, totalling more than $30,000.

Ms Priest says the ruling has left her unable to appeal.

"I'm so sad for Linda because I can't do any more," she said.

"I'm 72 years old and I wanted to try and give her some closure.

"I don't feel I can do that at the moment."