POLICE are investigating after more than 200 people were discovered at an illegal rave party near Dealba Road and Plantation Track in the Heathcote-Graytown National Park.
The incident occurred over the weekend of August 17-18.
Heathcote's Sergeant Matt Hunt said police were called to the park after a complaint about noise.
“We found a large group of people camping illegally," he said. "The site was set up with music stations together with speakers and strobe lights."
The following morning, State Highway Patrol conducted random drug and breath tests near the site.
"Five motorists returned a positive saliva-swab test for drugs," Sgt Hunt said.
"But it's was unlikely the event was organised locally. We spoke to a few people and they all came from various parts of Melbourne,” he said.
It is illegal to camp in a National Park without a permit.
Parks Victoria ranger team leader Mat Sobey said Parks will work with Victoria Police to investigate the unauthorised event.
"To ensure our parks are looked after, we require permits for events to make sure it is appropriate for the park landscape," Mr Sobey said.
"There are significant public safety and environmental impacts to consider when planning and participating in an event, which is why permits are necessary."
Mr Sobey said rangers assessed and cleaned up the saite.
"In the past, we have prosecuted people for holding unauthorised events. This should serve as a warning to anyone that organises an event in our parks to go through the proper process," he said.