Lake Tuggeranong, where a body was found floating last week.

Lake Tuggeranong, where a body was found floating last week. Photo: Supplied

Police have been unable to track down a key witness who may hold crucial information about a body found floating in the Tuggeranong Lake last week.

A 68-year-old Tuggeranong man was found floating unconscious in the lake on Thursday. He was pulled from the water by emergency services, but attempts to revive him failed and he died in hospital a short time later.

Investigators are yet to determine exactly what caused the man to enter the lake.

There were no signs of injury on his body, and police say they have found no evidence of foul play at this stage.

In their attempt to unravel the circumstances leading up to the incident, police were attempting to track down a potentially valuable witness.

Authorities were attempting to find a jogger who was running around Lake Tuggeranong at the time. But a public plea issued on Tuesday has so far been unsuccessful, and the witness, described as a young man about 165 centimetres tall, with short, messy dark hair, has not been found.

It was also revealed that the dead man's wallet was missing.

Detectives say his bank accounts have not been accessed inappropriately.

Police had still not found the wallet on Thursday afternoon.

Detective acting Sergeant Matt Lee said on Tuesday that the investigation would be "quite difficult" to move forward without more witnesses.

The victim is understood to have gotten off a bus at the Tuggeranong Bus Interchange about 12.35pm.

He walked past the La Porchetta restaurant, and was seen by the lake edge about 12.45pm.

Contact Crime Stoppers on 1800 333 000 or via the website