Police are yet to unravel the mysterious events leading up to the discovery of an unconscious man, who later died, floating in Lake Tuggeranong on Friday.

It has been revealed the man's wallet is missing, but his bank accounts remain untouched.

Investigators are seeking a witness, a young man who was running under Soward Way Bridge while the 68-year-old man was unconscious in the water nearby.

The unconscious man was noticed by a passer-by near the Soward Way Bridge about 12.55pm on Friday.

ACT Fire and Rescue pulled the man out of the lake. Paramedics tried to revive him but he died in hospital.

Criminal Investigations detectives are yet to establish a cause of death, despite an autopsy, but further tests are expected to be carried out.

Detective Acting Sergeant Matt Lee said the man's bank accounts had not shown any signs of unauthorised activity.

''We can't say whether the death is suspicious or not; we're calling for witnesses because an item of the male's property is still missing,'' Detective Sergeant Lee said.

''There's no evidence to suggest that foul play did occur, but obviously the family and [police] would like further evidence to rule out suspicious circumstances.''

The man, who lived in the Tuggeranong area, was wearing a distinctive black shoulder sling with a red ball attached to the end of it.

He was 170 centimetres tall with grey hair and an average build. He was wearing a blue and white checked shirt, grey shorts and brown sandals.

Police believe he got off a bus at the Tuggeranong Bus Interchange about 12.35pm and walked past the La Porchetta restaurant. He was seen standing by the lake edge about 12.45pm.

It is understood he had run errands in Tuggeranong that morning, but it is unclear why he returned to the area.

''[The man's family are] obviously very devastated; being close to Christmas, they're doing it pretty tough at the moment,'' Detective Sergeant Lee said.

He said there were likely to have been a large number of people around the lake during lunch on Thursday.

''At the time, the gentleman - from what we've heard - was just walking and standing at the lake, so there was nothing untoward going on, and [witnesses] may have just carried on about their business during the day unknowingly,'' he said.

Police are urging the runner, who may have been the last person to see the man alive, to get in contact with authorities.

The runner is described as a young man, about 165 centimetres tall, with short, messy dark hair.

Police believe he was wearing a peach-coloured top, dark shoes, and black shorts.
