When Tasmanian police arrived at the home of an elderly victim of armed robbery in 2001, they found only one piece of forensic evidence: a leech, freshly engorged with blood.

They bagged it and extracted the blood for DNA profiling, with little hope it would lead them to the culprit.

Last year, Peter Cannon was arrested on unrelated charges, and a cross-check established that his blood sample matched that taken from the leech.

In what is believed to be a world first, the evidence led to him pleading guilty this week to aggravated armed robbery.

It was, Detective Inspector Mick Johnston told the Hobart Mercury, "the oddest way of convicting anyone I have ever been involved in". He added: "I have not been able to find any similar cases anywhere in the world."

The leech was found next to a safe in the home of 71-year-old Fay Olson, who lives in a remote wooded area of Tasmania.

She had been threatened with sticks by two hooded men who broke into her house, tied her to a chair and stole 550 Australian dollars.

Police established that neither Ms Olson nor any of the officers who went to the house had been bitten.

Cannon, 54, who will be sentenced on Friday, faces up to 21 years in jail. Police are still searching for his accomplice.