But police say they don't need to ask for public assistance in most cases, and sometimes it can take weeks or months to determine that they need to appeal for help because they don't have enough evidence for a prosecution.

In the past six months, ACT Policing has regularly gone public with calls for witnesses or information several months after the incident under investigation took place.

Opposition police and emergency services spokeswoman Giulia Jones questioned this practice during annual report hearings in November, after three delayed appeals for help including the release of CCTV from an aggravated robbery that was more than eight months old.

"Why does it take 46, 180 or even 261 days to get information out to the public seeking their advice? Is this ideal?" Mrs Jones asked, referring to the time taken in the three examples.

Since then, ACT Policing has waited 71 days to call for witnesses to the alleged assault of a bus driver, and 60 days to appeal for witnesses to another alleged assault in Manuka. Both appeals were made in late February and followed incidents in December last year.

Dr Janie Busby Grant, a memory expert from the University of Canberra, said if police were appealing for information, they should do so as soon as possible to ensure they were getting reliable information.

She said memories became less detailed and specific over time, and people might even fill in details that had faded with more generalised, inaccurate information.

"Memory is reconstructed. It’s not like a video, so when we recall something we’re essentially patching it together and recombining elements to actually recreate the memory," Dr Busby Grant said.

"Over time, the details become harder to recall; they become less specific and you start slotting in things.

"It might be things that happen or it might be things that you’ve heard. For example, if they’re asking about a robbery and you’ve heard or read about a robbery in that intervening period, it would be very easy to accidentally slot in what you’ve heard since back into that original memory."

Dr Busby Grant said a person who regularly parked their car in the same place might remember parking it there on a day they didn't, simply because their memory told them it was normal to park in that place.

Asked whether police should look to engage the public as soon as possible if they planned to call for witnesses or information on a specific case, she said there was "no better recommendation than that".

"Basically, the closer to the event that it happens, the better," Dr Busby Grant said.

"But eyewitness memories are dodgy right from the get-go. In general, eyewitness testimony isn’t terribly reliable, even immediately after the event, and it gets worse from then on."

An ACT Policing spokesman said officers prioritised the collection of evidence and canvassed the areas around crime scenes for potential witnesses.

"As the investigation progresses and the circumstances are established, police may call for additional witnesses or information to support the investigation and ensure all possible avenues of evidence are explored," he said.

"In the majority of criminal cases, no public call for witnesses or assistance is required. Evidence and information collected during the investigation is sufficient for a prosecution to occur.

"However, in some cases, the information collected by the investigation (over weeks or months) may not be enough for a prosecution, so a call for additional witnesses or the release of CCTV footage may then be required."

The spokesman said the ACT Director of Public Prosecutions also assessed and validated witness testimony and evidence in order to proceed to trial.