One of WA's worst sex offenders has been sentenced to 12-and-a-half years in jail for what a Perth judge called the "cruel and degrading" sexual assault of a 13-year-old girl in Geraldton almost 25 years ago.

Patrick Alfred Merritt, who was formerly known as Patrick Comeagain, pleaded guilty to eight charges dating back to November 1994, when he dragged the girl from her bed before taking her to bushland and repeatedly sexually assaulting her.

Five days later, the then 21-year-old sexually assaulted a 9-year-old girl, a crime for which he was later sentenced to 10 years in jail.

He was not charged over the attack on the 13-year-old girl until 2016, after police conducted a cold case review.

Victim robbed of 'childhood, innocence and dreams'

District Court Judge Michael Bowden described Merritt's offending as "persisting and unrelenting".

"You took [the victim] from the safety and sanctity of her home into the terror of the night and subjected her to a cruel and degrading sexual attack." he said.

Judge Bowden highlighted parts of the woman's victim impact statement, which said the offences had "destroyed her life" and "stolen her childhood, innocence, dreams and hopes for the future."

He took into account the fact that Merritt had pleaded guilty and spared the woman from having to give evidence in a public court, but noted the admission only came on the day his trial had been due to start.

"[The victim] was ready to give evidence and had no doubt endured the stress of preparing for trial," Judge Bowden said.

Merritt behind bars for 22 of past 23 years: Lawyer

Sorry, this video has expired Dangerous sex offender dances when released from jail in 2014, but was soon back behind bars

Merritt was made for eligible for parole, but cannot be released after serving his sentence because he was declared a dangerous sex offender in 2008.

He is currently subject to an indefinite detention order, and under the Dangerous Sexual Offender legislation can only be released after an assessment by a Supreme Court judge.

In 2014, his release on a strict supervision order was approved and he was filmed dancing as he left Casuarina Prison.

The next year he was taken back into custody for breaching the order by taking drugs, and not reporting to his supervisor.

His lawyer told the court that out of the past 23 years, Merritt — who is now 44 — has spent only 12 months out of prison.