Homicide Squad detectives will allege a man arrested as he attempted to a board Bali-bound plane at Sydney Airport was the gunman who executed Bankstown lawyer Ho Ledinh as he sat outside a Bankstown coffee shop last month.

Strike Force Eugene investigators were forced to take the 38-year-old Sydney man into custody about 10am on Tuesday after being alerted that their chief suspect had bought a one-way ticket from Sydney to Indonesia and was about to board a plane.

It is understood that although the man was suspected of being the gunman, detectives were still working on gathering evidence when they had little choice but to pounce after an airport alert was triggered.

The man was taken to Mascot police station for questioning and was expected to be charged with murder on Tuesday.