Internal emails obtained under Freedom of Information laws show police are now attempting to use the cameras for purposes well beyond their initial design, raising alarm among some privacy experts warning of the potential for function creep.

According to the emails, in the past three years, police have made multiple requests to view camera footage in relation to crimes ranging from arson to home invasion, counterfeiting and high-profile investigations involving internationally protected persons.

Unlike point-to-point cameras, which record the details of every vehicle passing under their scanners, red-light and fixed speed cameras only record images when triggered, such as when a car runs a red light.

The introduction of point-to-point cameras in the ACT in 2012 led to heated debates in the ACT Legislative Assembly, with the opposition raising concern about the cameras being used for purposes other than monitoring speeding vehicles. Police have previously been reported as using the point-to-point camera networks, and even the MyWay bus ticket system, to track suspects, but are required under legislation to demonstrate such requests are "reasonably necessary for the enforcement of the criminal law". They are also required to make detailed, formal requests and to delete images within 14 days if they are not deemed relevant to investigations.

Australian Privacy Foundation board member Roger Clarke said police had been reluctant to reveal the exact nature of how they were using traffic footage. Mr Clarke said to protect the public's right to privacy, all images taken by traffic cameras should be deleted immediately, except where they had been detected in real time as having breached the law.

"For speeding and red lights, that means deletion immediately after capture. For point-to-point schemes, that means deletion immediately after the time it becomes apparent that no infringement has been detected," Mr Clarke said.

Police have also significantly expanded the number of cars capable of scanning number plates, up from an initial three to 14. But a spokesman said those images could not be used for tracking suspects, because they did not carry a location stamp.

The police spokesman said it was important to remember licence-plate scanners simply provided information to an officer, who then had to act on the information to determine whether infringements or other offences had been detected.

Privacy and surveillance expert and University of Canberra assistant professor of law Bruce Arnold said most reasonable people would accept traffic images being used by police for legitimate criminal investigations. But police and governments had not done enough to explain exactly how and why they were using the data.

"The challenge is one of optics. Governments typically promote this technology as traffic management, rather than broader law enforcement, and I suspect they do that because they are worried about pushback from some constituents, who say, 'wait a minute, this is getting into big brother territory," Mr Arnold said.

"We don't really have an informed public discussion about the capabilities of these technologies, and we are instead being told, 'trust us, we'll do the right thing', but they would get more trust if there was more openness."