Letter-writer key to Victorian cold case

An anonymous letter-writer holds the key to solving a 15-year-old murder mystery and Victoria Police wants them to come forward before they are "thrown under the bus".

Mum-of-three Kath Bergamin, 37, disappeared from her Wangaratta home after leaving the family farm and a tempestuous marriage.

Estranged husband John Bergamin and eldest son Stephen Bergamin are suspects and while a coroner concluded she was murdered, Ms Bergamin's body has never been found and the killer remains a mystery.

In April this year, a highly-detailed anonymous letter, which also asked for indemnity from prosecution, was sent to Victoria Police.

"You need to contact us, because you hold the key to potentially solving this matter," Missing Persons Squad Detective Inspector Tim Day said on Tuesday.

"It's clear from the letter the author has intimate details of what has happened, of who the offenders may be and intimate details of where Kath's remains might be."

Det Insp Day says anyone with information should consider their position.

"(Perhaps) through naivety and false pretences, the innocence of youth, they've been roped into certain events that at the time the gravity of which may not have been obvious to them," he said.

"Experience has shown us as investigators, that when we finally catch up with the principal offender, they attempt to mitigate and minimise their own culpability, often by throwing the non-principal offender under the bus."

Det Insp Day says while police have not talked to Mr Bergamin and his eldest son, they remain suspects. He could not reveal if they have talked to Ms Bergamin's younger children.

Modern DNA testing will be done on duct tape fashioned into a figure-eight that may have been used to restrain Ms Bergamin's limbs.

A red sock will also be tested to see if it matches fibres on the tape.

There is a $1 million reward and the possibility of indemnity from prosecution for information that solves the case.

Ms Bergamin split from her husband and moved off their Cheshunt farm in the months before her disappearance.

The couple had a "deeply troubled relationship" and Mr Bergamin would regularly resort to behaviour to deliberately frighten his wife, Coroner Peter White found in 2008.

Both John and Stephen Bergamin were excused from giving evidence at the inquest on the grounds they should not be compelled to give evidence that could incriminate themselves.