It's the common catchphrase used by law enforcement since the first "wanted" sketch posters were tacked outside a sheriff's office.

But further: would you recognise this person if they were wearing a mask, as is required almost universally under health protection protocols against the COVID-19 virus?

That's another, and far more difficult question to answer.

In Australia prior to 2019, the sight of someone wearing a face covering or a mask in public was unusual.

So unusual, in fact that if you were working late at night in a convenience store and someone came in wearing a mask, a common assumption was that you were about to become a victim of crime.

But in less than 18 months, perceptions of "rightness" and "wrongness" associated with face masks - and when it is, or isn't, appropriate - has been completely upended.

Now it's those not wearing masks in public who are seen as the outliers, and the mask-wearing practice is likely to be with us for a long time to come given the COVID virus variants which are now emerging in other countries.

Police easily admit that the universality of mask wearing has presented them with an additional challenge by turning one of their most useful tools - the so-called suspect "facefit", or facial identification - into little more than a mask with eyes.

Wearing a mask, hat and sunglasses, an offender could appear to hide in plain sight.

The science of creating a likeness of a criminal's appearance was first developed by French police officer Alphonse Bertillon as far back as 1882. The Bertillon system was a complex card-based indexed system which cross-referenced a standardised set of identifying characteristics.

The system was slowly refined over decades until, as expected, it became a digitised process and produced a highly accurate representation.

That image can now be fed into a global facial recognition system such as Clearview, co-developed by Vietnamese-born Australian and now US-based Hoan Ton-That.

Clearview is used by the federal police as well as hundreds of other police organisations around the world to scrape billions of images across the internet, making that search for the facial "needle in a haystack" far faster and easier. Even those criminals whose faces appear almost accidentally in the background of social media posts are detectable.

Detectives like acting Sergeant Beth McMullen, who works with the the crime team at Canberra's City Station, says the latest mask-wearing practice doesn't present as big an issue for police identification as the public might expect.

"Criminals have been wearing masks of some kind to hide their identity for a long time," she said.

"Police are accustomed to identifying people that cover their faces quite regularly; a mask might hide a small portion of a face but there are many other identifying features on each and every one of us."

She said that common identifiers were tattoos or piercings; skin, hair and eye colouring.

But the better the picture that can be built from a witness statement, the more useful it becomes. And this means police are seeking more nuances within those descriptions.

"We really want members of the community to make note of the physical detail of a person as much as possible such as their age and gender, hair colour, hairline, whether they have facial hair, the shape of their eyebrows, their height and build.

"And even things like how they talk and how they walk; does the person have an accent? Are they holding something? Are they left or right handed? We want people to think about what is really unique about this person they are describing."

It's understood that humans instinctively use more than just visual identifiers to recognise others but now with common mask-wearing, it's a skill we are going to have to hone and practise in the months ahead.

"In your everyday life if your family members are wearing a mask, you're still going to recognise them," acting Sgt McMullen said.

It could also be expected that with so much public mask-wearing, the value of CCTV as an evidence-gathering tool from a crime scene would diminish.

But the detective disagreed.

"I think it's a great time for business and people generally to be upgrading their CCTV because the quality of that vision - whether it's a person's build, physical attributes, the way a person walks - good CCTV can capture that and that could be really helpful for police to identify who that person is," she said.

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