Justin Laurens Stein has been jailed for life over the "execution"-style murder of nine-year-old schoolgirl Charlise Mutten in the Blue Mountains.

Stein has today been given the maximum sentence of life in prison after being found guilty of shooting and dumping the schoolgirl's body in a barrel near the Colo River, south-west of Sydney, in January 2022.

Stein was found guilty in June of shooting Charlise with a rifle in the back and again in the head from about 30 centimetres away.

During the trial Stein accused Charlise's mum, Kallista Mutten, who was his fiancée at the time, of murdering her own daughter.

Justice Helen Wilson told the NSW Supreme Court today Stein failed to demonstrate "any shred of remorse" for such a "shockingly callous crime".

"Having murdered a nine-year-old girl, all the offender's energy was spent covering his tracks and directing blame elsewhere," Justice Wilson said.

"Some instances of murder are so grave that the maximum penalty is the only appropriate penalty."

In her sentencing remarks, Justice Wilson spoke about how when giving evidence Stein's voice began to break and he appeared to cry.

"I could see very clearly ... he was completely dry-eyed and did not shed a tear," she said.

"It might have been called theatre if it wasn't so calculated."

The girl was murdered at Stein's family's Mount Wilson estate in the Blue Mountains while the two were alone.

The nine-year-old lived permanently with her grandparents in Queensland's Tweed Heads and was visiting her mum and Stein during the summer holidays.

Stein denied killing the girl but admitted to dumping her body in a plastic barrel, which was found near the Colo River after an extensive four-day search.

The court heard Charlise loved school, dogs, swimming and karate and had a close relationship with her grandparents, who she had been living with since the age of four.

At a sentence hearing on Friday, Justice Wilson said the sequence of gunshots in Charlise's killing was "almost akin to an execution".

Speaking before the sentence was handed down, NSW Police Detective Superintendent Danny Doherty said "very dedicated, determined investigators" helped bring the case against Stein to trial.

"It was horrendous circumstances and they were determined to get a result and they did," Superintendent Doherty said.

"I'm very proud of their efforts and they should be recognised and commended for their duty and their investigation."