A marked police van will be used outside major events or in crowded areas to live scan the faces of people walking past, in an Australian first.

The faces of people will be matched against a database of people with outstanding arrest warrants and registered child sex offenders, as well as missing persons.

"This is not about mass surveillance," [the police commissioner] said, "This is about specifying those in our community who are wanted by police."

While the trial only includes one marked van, the police commissioner is not ruling out further covert technology being used in the future, including at protests.