A man dressed in commando gear and carrying a semi-automatic gun was able to stroll unchecked through Bourke Street Mall in the days following the massacre that killed six people.

The troubling security breach – which occurred when Victorians were told authorities were on high alert to ensure the safety of Melbourne's CBD – is now subject to a full police investigation.

Police were only alerted to the man's presence after Fairfax sent them an image of the armed man.

They refused to explain why he was not approached earlier or why he has not since been charged.