A Supreme Court judge says a Frankston tip may have to be excavated to try and find the body of murdered schoolgirl Prudence Bird.

The Victorian Supreme Court heard convicted double murderer Leslie Camilleri has tried to help police locate the spot where he dumped the teenager's body 21 years ago.

Camilleri pleaded guilty to killing the 13-year-old girl after snatching her off the street near her home in Glenroy in 1992.

Camilleri approached police in May asking for maps of the Frankston area in Melbourne's south-eastern suburbs.

Police took Camilleri to an old tip site in Frankston in August and said he left the girl's body in an old wardrobe.

He told police he murdered a New South Wales man on the same night and also scattered his remains in the tip.

Camilleri told police he removed the distributor cap from the car he drove there to make it look like it had been abandoned, then cut a hole in the fence and went into the tip and disposed of the bodies.

Police told the court there are inconsistencies in his story.

For example he told them he went through scrub to get to the tip but it is next to a sports oval.

He also told police he could not see any houses, but it is a residential area.

Justice Elizabeth Curtain had urged Camilleri to was dumped.

But Camilleri's lawyer says he was only ever 80 per cent sure about the location, because he was not from Melbourne and had been taking drugs at the time of the incident.

While acknowledging the inconsistencies in Camilleri's story, Justice Curtain said there should be further investigation of his claims for the sake of Pru Bird's mother Jenny.

The court heard soil from the tip site is being tested.

Camilleri was due to be sentenced for Ms Bird's murder on Thursday but it has now been delayed.