Anyone attending Splendour in the Grass near Byron Bay later this month who is aged under 18 will now have to be accompanied by an adult.
Ticket holders have been told the new rule is due to a change in liquor licensing laws for the event.
It previously applied to people under 16 years of age.
Ticket company Moshtix sent out an email alert advising ticket holders about the changes.
"We appreciate that this is not ideal, we are not happy about this late change however we must adhere to these new conditions imposed on us by the Police and Liquor and Gaming," the notice said.
"Police will be present at the event, roaming throughout the crowd checking that underage minors are with a responsible adult."
The notice said there were still some "resale" tickets available for anyone who still needed an adult to accompany them.
Under NSW liquor laws, a responsible adult is defined as a parent or guardian, the minor's spouse or de facto partner, or person standing in as a parent for the time being.
Young fans may miss out
Sixteen-year-old Abbey Davies, from Lennox Head, has a three-day pass but does not expect her parents to purchase a similar ticket.
"Dad said he'll go for one day, to spend $200 on a one-day ticket," she said.
But said she has contacted festival organisers for a refund for the other days but is yet to hear back.
"There were about five of us going; four of us had three-day tickets and they're all in the same boat, no parents going.
"A couple of them just want to go anyway and hope no-one gets fined.
The Moshtix email says those unable to find a responsible adult to attend the event can contact a support website using "SITG – U18" as the subject line to have any ticketing request reviewed.
The ABC has contacted Splendour in the Grass and NSW Liquor and Gaming for a response.
