A “vigilante” who poured boiling water over a notorious murderer in a West Australian prison has failed in a bid to have her five-year jail term reduced.

Nyiltjiri Naalina Forrest was disgusted by Trudi Lenon’s 2016 crime, which involved her luring autistic teenager Aaron Pajich to “thrill killer” Jemma Lilley’s Orelia home where they garrotted and stabbed him.

Forrest’s attack on Lenon last year left her with second degree burns to 21 per cent of her body.

The sentencing judge noted any burn greater than 15 per cent of the body surface area was potentially life-threatening, but the quick actions of prison officers helped ameliorate the harm Lenon suffered.

She was in hospital for about three weeks, including two days in an intensive care unit.

Camera Icon Trudi Lenon was left with second degree burns to 21 per cent of her body after a fellow prisoner threw burning water over her. Credit: Facebook

Forrest, who completed her previous jail term, unsuccessfully argued in the WA Court of Appeal the additional sentence was excessive, with her counsel drawing a parallel between her traumatic childhood and Lenon’s crime.

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At age five, Forrest helped her father bury a body in a backyard and Mr Pajich was also buried in a rear garden, but this was not raised with the sentencing judge.

Forrest’s attack on Lenon was deemed premeditated given she had repeatedly requested a transfer, saying she could not be near people like Lenon, Lilley and serial killer Catherine Birnie because she was scared of what she might do to them.

Forrest said she didn’t mean to also burn another murderer, Justine Campbell, who was splashed by some of the water.

Forrest must serve at least three years behind bars before she can be eligible for parole.