Teenager badly burned at Sydney party after agreeing to be set alight

A young man who allegedly agreed to be set alight with flammable liquids at a party in Sydney's north is in hospital with burns to almost half his body.

Footage shot by another party-goer just after midnight shows Northern Beaches teenager Johno Holten wearing no pants and allowing himself to be sprayed with accelerants before another person lights his leg on fire.

Not satisfied with performing the dangerous stunt once, the 18-year-old asked his friend to use lighter fluid on him.

The fire quickly spreads up his leg and onto his back and torso.

The prank was apparently consensual but moments after the teenager's friends began desperately trying to put out the fire when it spread to his clothing.

Brendon Mackie was at the party and said it was "just a harmless night at the start".

"He was passed out, and then woke up yelling 'light me on fire, light me on fire', stupidity basically," he said.

Mr Holten sustained excessive burns to 45 percent of his body and was taken by ambulance to Royal North Shore Hospital in a stable condition.

St Vincent's Hospital director of emergency services Professor Gordian Fulde said testosterone and peer pressure would contribute to the problem.

"The excitement of it all, if you add alcohol to it you have a potential fatal combination," he said.

Officers attended the party venue at Collaroy Plateau and arrested 18-year-old Kane Forrest for questioning at Dee Why Police Station. He was released without charge.