Police said a 30-year-old man was assisting with their inquiries but no arrests had been made.

ACT Policing arrived at about 2am in the vicinity of Hall Best Lane and Wizard Street, Gungahlin following reports of an altercation in the victim's home.

Police located the 56-year-old man who had suffered critical injuries. The man died at the scene as police conducted CPR.

Police have ruled out a shooting. They have not determined the cause of the man's death, but are treating it as suspicious. Detectives have not located a weapon.

Detective Superintendent Scott Moller said the victim had significant critical injuries.

"We're unsure at this time exactly what they are. He suffered significant blood loss," the detective said.

"It's a horrific attack. A male has died during this incident, so it's something that ACT Policing take very seriously and put all of our resources into.

"We'll be here for a number of hours processing these scenes, and we'll keep working through it till we find out what actually has happened."

Officers closed part of Wizard Street and parts of Hall Best Lane during their investigations. Among other crime scenes were a residence and a vehicle.

Forensics officers were seen laying markers on Hall Best Lane on Monday morning. Witnesses reported seeing police inspect the lane at about 8am, when officers also interviewed residents. At 12pm police on nearby Hamer Street inspected drains as they searched the area.

Police asked anyone who might have any CCTV or dashcam footage, or information that could help them, to contact Crime Stoppers on 1800 333 000 or at the Crime Stoppers ACT website and quote reference number 6372686. They said information could be provided anonymously.