A man who abducted a four-year-old child and held her captive at his home for more than two weeks has lost his appeal to have his sentence reduced.

Terence Kelly abducted Cleo Smith from her parents' tent at a remote West Australian campsite north of Carnarvon in late 2021, sparking a massive manhunt before she was found at his home by police 18 days later.

Today's decision, in the Court of Appeal, will see his original sentence of 13 years and six months in jail stand.

Kelly was not present in court today as the decision was handed down.

Mental impairments cited

He was jailed in April 2023 after admitting he kidnapped Cleo, but his lawyers argued his mental impairments, including severe personality dysfunction, contributed to his decision.

They also maintained the sentencing judge erred in finding Kelly's use of methamphetamine had a "significant and causal role in the offending".

This was, Kelly's lawyers said, because she gave greater weight to witness statements about whether Kelly was on meth when he abducted Cleo, over expert evidence provided by a doctor.

They also say the judge didn't give enough consideration to Kelly's childhood disadvantage and trauma.

Prosecutors had opposed the appeal, arguing the sentencing judge's findings were consistent with the evidence before her and that she had appropriately considered Kelly's impairments and deprived upbringing.

In sentencing, WA District Court Chief Judge Julie Wager said Kelly's actions were at the "highest level of seriousness" and that Cleo and her family would be "permanently impacted".

But she acknowledged Kelly's upbringing and outlined how he was exposed to severe and complex trauma as a child and suffered neurological impairment.

She accepted he had turned to drug abuse due to the pain caused by that trauma.

Reasons for decision

Cases that go before the WA Court of Appeal – the state's highest court – are considered by a panel of three Supreme Court justices.

While all justices agreed unanimously on three of the four grounds of appeal, the president of the court, His Honour Michael Buss, said he would have granted Kelly's appeal on the ground that the sentence was "manifestly excessive".

But he was overruled by the other two.

In its ruling, the court accepted the sentence was "severe" but found it reflected the seriousness of the crime.

"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," Justices Robert Mazza and Stephen Hall found.

"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."

Kelly had faced a maximum of 20 years in jail but he got a 25 per cent discount of five years for his early guilty plea.

The court noted other mitigating factors including his mental impairments and profound childhood deprivation had resulted in a further reduction of his sentence of one-and-a-half years.

The appeal court found this was appropriate.

"In our opinion, the sentence that was imposed was proportionate to the seriousness of the offence," Justices Mazza and Hall wrote.

"When weighed against the extraordinarily serious circumstances of the offence, the need for protection of the public, and having regard to the discount that was given for the plea of guilty, the weight that could be given to other mitigating factors was relatively limited."

Abduction sparked huge search

Kelly's abduction of Cleo hit international headlines and caused uproar in the small WA town of Carnarvon, some 900 kilometres north of Perth.

Her parents woke at the popular camping spot The Blowholes, 50km north of Carnarvon, on October 16 to find the zip on the family tent open and Cleo and her sleeping bag gone.

It sparked one of the biggest land and sea searches in the state's history, but she was ultimately found through police ingenuity and data analysis of phone tower traffic in the area.

Kelly's social media profiles revealed a fantasy family life in which he represented himself as married with children, but family members who spoke to the ABC in the aftermath of his arrest described him as "quiet" and "sticking to himself".

The ABC has chosen not to name those people, who said they were shocked by his crimes.

They said Kelly lived alone and didn't socialise much, particularly after his grandmother died.

He was raised by the grandmother, and she was "the only one he had", family members said.

Kelly's social media profiles included prolific posts about dolls and photos of himself posing with his collection.

The sentencing judge described him as living in an "idealised fantasy world" which protected him from the real world.