Several people tried to put out the flames with their hands to stop him from burning, said Gilmore.
The party was supposed to be a small celebration for close friends of his 17-year-old flatmate, but he said about 50 people suddenly turned up uninvited.
"All of a sudden it blew up out of control."
The victim ran from the party to his home in the nearby suburb of Akina, where police and emergency services were called about 1.40am.
He had suffered burns to his back and neck and was taken to the intensive care unit at Hawke's Bay Regional Hospital.
Yesterday afternoon he was in a stable condition, said hospital spokeswoman Chris Lord.
A male, aged 17, has been arrested and charged with wounding with intent to cause grievous bodily harm.
"I can't rule out that there won't be other people arrested," said Detective Sergeant Mark Moorhouse.
The motivation behind the attack was unclear, he said.
"I think it was young men being very stupid. "
As part of the investigation, police would speak to the victim and the party-goers, "which can take some time".
Treatment of petrol-related burns involved multiple operations, skin grafts, pain and weeks of healing and rehabilitation, said New Zealand Association of Plastics Surgeons president Dr Howard Klein.
Grafts were taken from a non-burn area, typically the thighs or abdomen.
"It can be life-changing and it is horrifically painful," Klein said. "Petrol is a direct-flame burn so with the accelerant on the skin you have a greater chance of having a deep burn."
Hastings City Business Association general manager Jane Janes called the attack "horrific".
