Terence Darrell Kelly attempted to appeal the 13-and-a-half-year jail term he received for snatching the girl at the Blowholes campsite, about 70km north of Carnarvon in the early hours of October 16, 2021, as her parents slept.
Cleo was missing for 18 days before being found by police alone in a room at a property in Carnarvon on November 3.
Cleo Smith's kidnapping sparked one of the biggest police searches in WA history. (Richard Wainwright/AAP PHOTOS)
Her kidnapping by the 37-year-old sparked one of the biggest police searches in WA history and made headlines worldwide.
Under his sentence handed down in the District Court in April 2023, Kelly will be eligible for parole after serving 11 years and six months after he pleaded guilty to abduction.
In February, his lawyers argued four grounds for appeal in the WA Court of Appeal.
The first alleged Chief Judge Julie Wager erred in law and fact in finding that the appellant's use of methylamphetamine had a significant role in his offending.
The second and third grounds alleged Chief Judge Wager erred in law in applying two principles of law, including by failing to sufficiently acknowledge his deprived, traumatic childhood and mental impairment when assessing his moral culpability.
The three appeal court judges dismissed grounds one, two and three in a Perth court on Monday.
"Her Honour did not misapply any legal test applicable to the determination of whether the appellant's use of methylamphetamine had a significant and causal role in the offending," Justice Michael Buss said in the decision.
The three judges disagreed on the fourth ground of appeal claiming the sentence was manifestly excessive, with Justice Buss finding it had been made out and that he would have reduced Kelly's sentence to 12 years due to the mitigating factors.
Cleo was found after 18 days in a room at a Carnarvon property. (Richard Wainwright/AAP PHOTOS)
Justices Robert Mazza and Stephen Hall came to a different conclusion, dismissing ground four.
"On any view, the appellant's abduction of such a young and highly vulnerable child from her parents, at night, and then holding her captive in his house for 18 days was extraordinarily serious," they said.
"As tragic as the appellant's background is, the sad fact remains that his risk of reoffending required that the sentence imposed upon him have regard to the sentencing objective of public protection."
Justices Mazza and Hall said Chief Judge Wager faced a difficult sentencing exercise and Kelly's circumstances were complex.
They said Kelly's mental impairments and profound childhood deprivation were factors that "pulled both ways" and that his methylamphetamine use contributed to the offending.
"The appellant poses a well above average risk of reoffending and, should that risk eventuate, there is a high risk he would inflict serious psychological harm on any future victim," they said.
The justices said the head sentence of 20 years' imprisonment, with discounts for the guilty plea and mitigating factors, was justified due to the seriousness of the offending and need to protect the public.
"The sentence imposed upon the appellant was severe but it was an appropriate reflection of the extraordinarily serious nature of the offence the appellant committed," they said.
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