Heathcote Leading Senior Constable Stewart Probst said police were called to the park about 9pm on Saturday.

He said they found hundreds of people camping illegally in the park.

Police pulled over eight cars as they were leaving the site and tested drivers for drugs and alcohol.

Four of the eight drivers were found to be driving with amphetamines in their system.

Leading Senior Constable Probst said those people were banned from driving for 24 hours.

He said they could be charged on summons pending a toxicology report.

Read more: Bendigo's national parks 'neglected'

Parks Victoria said it was illegal to camp at the park without a permit.

"Parks Victoria will continue to work with Victoria Police to investigate the unauthorised event held in the Heathcote-Graytown National Park last weekend," Parks Victoria ranger team leader Mat Sobey said.

"To ensure our parks are looked after, we require permits for events to make sure it is appropriate for the park landscape.

"There are significant public safety and environmental impacts to consider when planning and participating in an event."

Mr Sobey said rangers would assess the site and clean up where necessary.

"We are focused on protecting and conserving Victoria's natural environment and ensuring visitors to our parks have a safe and enjoyable experience," he said.

"In the past, we have prosecuted people for holding unauthorised events which should serve as a warning to anyone that organises an event in our parks without going through the proper process.

"We urge those enjoying the great outdoors to respect the natural environment."

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