A teen who threw a cup of petrol at his ex-girlfriend, causing burns to 20 per cent of her body, will spend at least two years behind bars.
Judge Paul Conlon rejected the boy's account of how the girl came to be doused in the fuel the morning of July 12, 2013, at an Albion Park Rail house.
"It was not difficult to determine the true facts of this matter," the judge told Wollongong District Court on Thursday morning.
"In my view the offender was a most unimpressive witness and I have rejected his version in its entirety."
The boy, who was 17 at the time of his offending, had pleaded guilty to a charge of recklessly causing grievous bodily harm but disputed the Crown's allegation that he threw a cup of petrol at the then 19-year-old, instead saying he had only kicked the cup in her direction.
Judge Conlon considered differing accounts of where the boy was standing when the cup's contents were dispensed, before ruling the boy's version would have made it "virtually impossible" for the petrol vapours to come near enough to an open fire to have caused the girl to ignite.
A witness earlier told the court the boy was heard to say "she's on fire, she's on fire, what have I done?" in the moments that followed.
The girl was placed in a coma for eight days and underwent multiple operations aimed at healing burns to her neck, chest, stomach, thighs, arm and hand.
Judge Conlon acknowledged the victim's "extreme pain and suffering" and her likely ongoing psychological and emotional injuries.
He considered the boy's lengthy criminal record as well as a report detailing his early exposure to drug use and his premature departure from the education system.
The boy had fathered a child when he was 14, the court heard, and acknowledged using the drug ice in the four days leading to when the girl caught fire.
A court-appointed psychologist described him as "a most unreflective young man" who regarded himself as "a bit of a loose cannon" and "lost".
Judge Conlon ruled that the boy could see out his prison term in a juvenile justice centre because, if placed in an adult prison, he would be too easily influenced by other offenders and wouldn't have the same education options.
He sentence the teen to three years' prison, with a two-year non-parole period. He will be eligible for release on July 13, 2015, at the earliest.
Outside the court, his victim told the Mercury the sentence was adequate.
"It is for me - I'm alive," she said.
"I could have died, so anything is a bonus for me.
"I'm happy that it's over and I get to move on with my life."
The girl told the Mercury her burns were "all healed now".