According to YouTube user "metalmatalbe", a second flare was launched during Bring Me The Horizon's set, landing on the top of the stage.
See: A flare being let off in a moshpit at Soundwave.
Soundwave promoter AJ Maddah appealed on Twitter for concert-goers to send him any photos they had of the person responsible.
"If we catch anyone w/ flares you'll not only be evicted/arrested, I'll dedicate every iota of energy & money I can muster to ruin your life," he wrote.
Maddah said the flare "seriously burned and disfigured" the girl.
"Poor little thing is really hurt. Massive awful burn on her forearm. I am devastated for her."
Soundwave, which was this year headlined by Metallica and featured many of the world's biggest metal, punk and rock acts, has overtaken the Big Day Out as the largest music festival in Australia, with 270,000 tickets sold and all five national shows selling out, the Sydney Morning Herald reported.
About 70,000 people were at last night's festival at the Sydney Olympic Park.
- nzherald.co.nz