More than 20 people have been hospitalised after overdosing at a Melbourne music festival.

Overall ambulance officers treated 30 people who attended the Electric Parade Music festival, with 22 of them being hospitalised after taking what was believed to be the synthetic drug GHB at the Myer Music Bowl.

Many of those treated were described as being in a critical condition at an event that was described by Ambulance Victoria State Health Commander Paul Holman as "awash with drugs".

"The majority of those treated by paramedics had overdosed on GHB," Holman said.

"We have transported 22 young people from that event alone and we're now up to 30-plus from that event and across the city all with GHB overdoses, all critically ill.

"It's the highest number of overdoses we have seen at a music event for some time."

"That we haven't seen (any deaths) tonight is probably just good luck and not anything else."

The event is scheduled to head north today and will take place at the University of Sydney.

© AAP 2017