NSW District Court Judge David Wilson on Tuesday in Gosford said mechanic Troy Johnson, 33, had engaged in "truly evil conduct" when he attacked the girl as she walked to school.

Judge Wilson, who lifted an earlier suppression order that had prevented Johnson being named, said the girl "was truly a victim of the detestable conduct of the perverted offender".

Johnson pleaded guilty to seven charges involving the sexual assault of the 12-year-old girl on May 15, 2017.

He also pleaded guilty to three charges involving the grooming and sexual abuse of another girl, aged 11, between August 2016 and April 2017.

He was jailed for 28 years with a minimum of 21 years.

The 12-year-old girl had been walking to school along a bush track on the Central Coast when she looked back and saw Johnson - who had told a friend he fantasised about raping somebody - suddenly emerge from the bushes.

Dressed in a camouflage outfit known as a leafy ghillie suit, Johnson dragged her into nearby bushland, tied her to a tree and sexually assaulted her for almost an hour.

Johnson, holding a knife in his right hand, told the girl, ''Shhh, don't scream. Don't scream or I'll have to hurt you."

When he eventually cut the girl loose, she ran crying to school. She was hysterical.

The judge said after raping the girl, Johnson called a friend and told him, "Hey dude, you'll never guess what I did".

Johnson boasted about how he had tied up a schoolgirl and sexually assaulted her, and talked about how he had used cable ties, duct tape, a knife and camouflage clothing.

The judge rejected Johnson's claims he deserved a lesser sentence because he had been of good character before the rape after it was discovered he had hidden a video camera in his family home since 2009 to film unsuspecting women in the bathroom.

Judge Wilson pointed out Johnson had stalked the schoolgirl for weeks before carrying out his abhorrent plan to rape her.

The judge believed Johnson had lied to his psychiatrist after his arrest when he claimed the attack was out of character, he did not know why he had done it and blamed the devil.

Judge Wilson had warned him that because he was too cowardly to enter the witness box to express remorse and admit his paedophilic behaviour, and because of the devastating impact it had had on his victim, his jail sentence would be harsh.

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