An anonymous letter sent to police about the disappearance of a Melbourne mother-of-three is believed to have been sent by someone who holds the key to solving the murder mystery.

Kath Bergamin, 37, disappeared without a trace from her Wangaratta home on August 18, 2002.

A figure eight of duct tape, which police believe was used to restrain Ms Bergamin, was found on her pillow in her bedroom after she vanished, reports news.com.au.

Her body was never found but a coroner later concluded she was abducted and murdered. It was not determined who had killed her. At the time, police suspected Kath's estranged husband John — who she had split from prior to her disappearance — was involved. But they couldn't prove it.

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A string of other leads resulted in dead ends. Until an anonymous letter with information about Ms Bergamin's murder was last year sent to police — 16 years after she vanished.

On Friday, police launched a fresh appeal for the author of the letter received by authorities in March 2017 — with "very specific information about Kath's murder, including who was involved" — to come forward.

The figure eight duct tape police believe was used to restrain murder victim Kath Bergamin. Photo / Supplied
The figure eight duct tape police believe was used to restrain murder victim Kath Bergamin. Photo / Supplied

Officer in charge of the missing persons squad, Detective Inspector Andrew Stamper, said he was reasonably confident the letter, which has a barely visible 'BE' followed by a series of illegible numbers written on it, was sent from Bendigo, Benalla, or the areas in between, the Bendigo Advertiser reported.

"This letter tells us that there are people out there that know she is dead, know who killed her and know where she is and it's troubling them," he said in a press conference.

"I think that the weight of their knowledge of this matter is weighing on them significantly and they'll want to get it off their chest and that can be a very difficult process."

A close-up shot of a scrap of duct tape found on a pillow in the bedroom of murder victim Kath Bergamin. Photo / Supplied
A close-up shot of a scrap of duct tape found on a pillow in the bedroom of murder victim Kath Bergamin. Photo / Supplied

Missing persons squad boss Tim Day last year confirmed there was "material in the letter that suggests they have either been told what happened to Kath or they were there when she was taken", the Herald Sun reported.

Ms Bergamin fled from the family home in Upper King River Rd, Cheshunt, on May 29, 2002, and sought an intervention order against her husband. Her children stayed with him.

A red sock found in the hallway of murder victim Kath Bergamin's home following her 2002 murder. Photo / Supplied
A red sock found in the hallway of murder victim Kath Bergamin's home following her 2002 murder. Photo / Supplied

She commenced legal proceedings to get financial support from her husband — and a share of the family business and farm — in mid-August 2002.

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She disappeared just a few days later. Police charged Mr Bergamin in 2004 with murdering his wife, but the charge was later dropped following legal advice from the Director of Public Prosecutions.

A $1 million reward remains on offer for information that solves the case.