BEGA schoolgirl killer Leslie Camilleri has pleaded guilty to murdering a Melbourne teenager who went missing more than 20 years ago.

Prue Bird disappeared from her Glenroy home on February 2, 1992.

Her mother, Jenny, trembled and sobbed as Camilleri, 43, pleaded guilty to murdering her 13-year-old daughter in the Victorian Supreme Court today.

The court heard that while Camilleri admitted to committing the murder, on or about February 2, 1992, he disputed some of the facts put forward by prosecutors.

His lawyer Jacqui Kennedy said Camilleri denied the prosecution's assertion that he acted in concert and insisted he acted alone.

"It is still put that he acted in concert with another person. That is disputed," Ms Kennedy told Justice Elizabeth Curtain.

Prue disappeared from her home after her mother left her in the care of a friend.

The friend left the house briefly to pack boxes in the garage.

Police had previously alleged that Prue was murdered as payback for evidence given by a key prosecution witness over Melbourne's 1986 Russell Street bombing, which killed a police officer and injured 21 others.

Prosecutor Michele Williams SC told the court the crown would no longer rely on material relating to the Russell Street bombing as a motive for the teenager's murder.

The matter will proceed to a contested plea hearing on February 4.

Prue's body has never been found.

Police conducted a three-day search for her remains near Flat Rock Creek, north of the town of Cann River in far East Gippsland, in February.

But the search was called off after officers found nothing.

The site is near where the bodies of schoolgirls Lauren Barry and Nichole Collins were found in 1997.

The girls were kidnapped by Camilleri and an accomplice in Bega, NSW, and driven across the border to the isolated region where they were repeatedly raped and killed.

Camilleri is serving life without parole for those murders.