Stein was found guilty of murder in June, despite his claims it was the girl's mother Kallista Mutten - with whom he was in a relationship - who shot the nine-year-old.
Justice Helen Wilson is due to deliver his sentence in the NSW Supreme Court on Monday, having previously said the girl's young age and callous manner of her death were significant factors to consider.
Police located Charlise's body on January 18, 2022, near the Colo River, northwest of Sydney, with gunshot wounds to her face and lower back.
A community grieved for Charlise Mutten, nine, whose body was found in a barrel dumped in a river. (Regi Varghese/AAP PHOTOS)
Justice Wilson said it was possible Stein shot the girl once while she was trying to flee, before approaching her and firing another shot directly into her head.
"It's almost akin to an execution," she said during his sentence hearing on Friday.
Crown prosecutor Ken McKay SC said due to the extreme nature of the crime the only appropriate sentence was life in prison.
"He went to great lengths to avoid being held responsible for his actions," Mr McKay said.
Stein's lawyer Carolyn Davenport SC, said it would be "very cruel and unusual punishment" to send a man of Stein's age to prison for the rest of his life.
"There was no motive that we know of," she said.
Charlise had been visiting her mother and Stein for Christmas from the Gold Coast, where she lived with her grandparents.
She spent the night of January 11 alone with Stein at a property in the Blue Mountains, while her mother stayed at a caravan about a 90-minute drive away.
Charlise's mother Kallista Mutten was in a relationship with killer, Stein. (Bianca De Marchi/AAP PHOTOS)
In a statement to the court, Ms Mutten said by placing her trust in Stein she had unknowingly put her daughter in harm's way.
"She trusted in my judgment and I just hate myself for being so wrong about it," she said.
The same day Charlise's body was found, investigators arrested and charged Stein with her murder after using location data from his phone to pinpoint where the barrel was dropped.
He would later admit dumping the body but claimed he panicked after discovering it on the back of his ute.
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