Mystery still surrounds the discovery of the body early on Thursday afternoon. Police were yet to formally identify the man and had few clues to suggest how he ended up in the lake.

The man, believed to be in his 60s, was discovered floating unconscious in 1.5-metre deep water near Soward Way Bridge in Greenway before 12.55pm. A passer-by spotted his body and alerted emergency services.

Police arrived at the scene, and firefighters pulled the man from the lake, before intensive-care paramedics attempted CPR. He was rushed to Canberra Hospital unconscious.

Attempts to revive the man failed, and he was pronounced dead in hospital before 3pm.

Police have told Fairfax Media there were no obvious signs of injury on the man's body.

Criminal Investigations Detective Sergeant Craig Marriott said there was nothing to suggest the death was ''untoward''.

Suicide has not been ruled out, but police are still seeking more witnesses. The passer-by who discovered the body was the only witness police had to date. They are calling for anyone who saw the man either in the water, or on the shores of the lake in the hours beforehand.

Detectives were making identification of the body and making contact with the family their top priorities on Thursday evening.

''The first step is identification, to notify next of kin, and try and obtain some details on the circumstances that have led to him being at the lake in the first place,'' he said.

A post-mortem is expected to be conducted in the next two days. The body was found close to the edge of the lake, on the banks closest to the Tuggeranong town centre.

Anyone with information should call Crime Stoppers on 1800 333 000, or via www.act.crimestoppers.com.au. Information can be provided anonymously.