Sexual predators can move freely between youth organisations by exploiting nationally inconsistent child protection laws and loopholes in background checks, a royal commission has heard.

Giving evidence on the seventh day of a public inquiry into the Australian Defence Force, retired Air Commodore Dennis Green agreed the situation created risks for youngsters.

Commodore Green is a former director general of the Australian Air Force Cadets, where the commission has heard a number of teenagers suffered sexual abuse.

"The legislation across the states and territories relating to child abuse is not consistent," he said.