Melissiah Diabel's mother Louise Faulkner and half-sister Charmian were last seen leaving their St Kilda flat on 26 April, 1980 after Louise told friends she was going to spend the weekend with her boyfriend George Sutherland, formerly of Parker's Corner, north of Erica.

A police investigation at the time found nothing and the disappearance of Louise and Charmian became a cold case.

Mr Sutherland, now in his 80s, moved to Tasmania.

The case was reopened in 2001 and a coronal inquest into the deaths of Louise and Charmian was held in August 2006 and this month.

But the inquest's findings, released on Tuesday, have also failed to provide any answers for Ms Diabel.

Deputy state coroner Iain West found that Louise and Charmian Faulkner were deceased and that "they met their death by foul play" but "in the absence...of `clear, cogent and exact proof'," a person or persons responsible could not be identified.

The coroner found the deaths of Louise and Charmian occurred "on an unknown date, at an unknown place, from an unknown cause".

However Ms Diabel has criticised the findings of the coroner's report and said not all the evidence was properly considered.

"I'm disappointed because there are a lot of mistakes, you would think a coroner should be able to get it right," she said.

"He hasn't looked at the facts presented to him.

"It's a bit like Jaidyn Leskie, they had to keep going back to get the truth."

Ms Diabel believes Mr Sutherland has something to do with her mother's disappearance.

She said initial police investigations into the case were "taken very poorly" and evidence given at the inquest about the physical appearance of Mr Sutherland at the time of Ms Faulkner and Charmian's disappearance, was only taken from two people, who gave conflicting accounts.

Ms Diabel said her own information about Mr Sutherland's appearance and evidence from her grandmother, had not been taken into account.

The coroner said Ms Faulkner's flat had been searched after the missing person's report had been made and "the only item that appeared to be missing was a television that had been a gift from George Sutherland...", but Ms Diabel insisted other items were missing and these had not been considered in the coroner's finding.

Ms Diabel and her family have spent the last 18 months making trips to Erica and Parker's Corner to search for clues and interview people who remember seeing her mother in the area at the time.

She said local residents were very helpful and she would continue to make the trips until she found all the answers.

"I have to get the message across that just because there has been a coroner's finding, this investigation is in no way closed and I will keep going until I get justice," Ms Diabel said.

She believes people would be more forthcoming with information if there was a reward to help solve the case, and has called on Victoria Police Chief Commissioner, Christine Nixon, to explain why one has not been offered.

"I have been lobbying the police for a substantial reward, I have been asking for years for a reward and have not been satisfied with the answers," she said.

A spokesperson for the Chief Commissioner had not responded to a request for comment at the time of going to print.

In a bid to solve the mystery of her family's disappearance and others who have lost loved ones, Ms Diabel started the `Louise and Charmian Foundation' for missing persons.

She has recorded a CD with two songs dedicated to her mother and half-sister, with all proceeds going towards the foundation. For details go to www.louiseandcharmian.com.