Three people have been treated for burns after a flare was let off at a rock concert in Sydney.

Police say the revellers were burnt when the flare was fired in the moshpit during the Soundwave event at Olympic Park, in the city's west yesterday.

The concert's promoter AJ Maddah took to Twitter after the incident calling for people to identify the person who set off the flare and offering free tickets for information.

"Tonight you seriously burned and disfigured a young girl," he said.

"Poor little thing is really hurt. Massive awful burn on her forearm. I am devastated for her."

He later tweeted, "thank you Twitterverse we now have images of this jerk from multiple angles. Hopefully we can post ASAP and get him identified."

About 75,000 people which featured music heavyweights Metallica and the Offspring.

Almost 900 people were treated for minor injuries at the event and 13 people had to be treated in hospital.

More than 50 people were caught with illicit drugs including cannabis, ecstasy, ice and amphetamines.