Western Australia's most violent criminals may be kept behind bars beyond their sentence under new laws operating the same way dangerous sex offenders can be kept in jail indefinitely.

The legislation passed on Wednesday night and comes into effect on September 1.

Attorney-General John Quigley said the Director of Public Prosecutions or State Solicitor's Office could ask the Supreme Court to order continuing detention or supervision of a seriously violent offender who continues to pose an unacceptable risk of reoffending at the end of their sentence.

"This will include members of outlaw motorcycle gangs, offenders convicted of kidnapping, robbery, and grievous bodily harm, as well as additional sexual offences including those involving child exploitation material."

AAP