More than 20 people have been taken to hospital, many in a critical condition, after overdosing on drugs at a dance party in Melbourne.

Twenty-five people were treated after taking what is believed to be the synthetic drug GHB during the Electric Parade Music Festival at the Sidney Myer Music Bowl on Saturday night.

What is GHB? Gamma hydroxybutyrate is a synthetic drug usually taken as a colourless, odourless liquid that tastes slightly salty

Gamma hydroxybutyrate is a synthetic drug usually taken as a colourless, odourless liquid that tastes slightly salty Effects including euphoria, increased sex drive and lowered inhibitions can be felt within 20 minutes

Effects including euphoria, increased sex drive and lowered inhibitions can be felt within 20 minutes Also lowers temperature and heart rate, and can cause memory lapses, drowsiness, dizziness and headaches

Also lowers temperature and heart rate, and can cause memory lapses, drowsiness, dizziness and headaches Overdose symptoms include vomiting, sweating, irregular breathing, confusion and agitation, hallucinations, blackouts and seizures

Overdose symptoms include vomiting, sweating, irregular breathing, confusion and agitation, hallucinations, blackouts and seizures Alternatively known as fantasy, grievous bodily harm (GHB), liquid ecstasy or fishies

A fleet of ambulances took 21 people to the Alfred, Royal Melbourne and St Vincent's hospitals.

Another seven people were treated after leaving the event and collapsing in nearby parks and streets.

Police arrested 40 people at the festival, including 33 men and seven women, while 28 people received diversions and two were given cannabis cautions.

One man was charged after ecstasy, cocaine, MDMA, LSD, ketamine and hash were found on him.

He was remanded into custody.

'There's always a risk they're going to die'

Paramedics said they were disappointed in what they called the highest number of overdoses seen at a music event for some time.

"This is particularly disappointing on a night when Melbourne's out enjoying itself," State Health Commander Paul Holman said.

"Really, that event was awash with drugs.

"There's always a risk that they're going to die, and that we haven't seen that tonight is probably good luck rather than anything else."

Mr Holman added the drug could cause people to fall unconscious by slowing the heart and causing seizures.

No plans to introduce pill testing, minister says

Mental Health Minister Martin Foley said the State Government had no plans to introduce drug (or pill) testing, but conceded harm reduction efforts had not been effective.

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"This was a tragedy of the proportions that we have not seen when it comes to drug overdoses at these kind of events," he said.

"What we need to do is ramp up our harm reduction efforts, the $192 million that we spend on all sorts of programs around peer support work.

"Getting the right messages about safe behaviour are clearly not working and we will consider how we have to do more."

He said the private event had nothing to do with nearby White Night festival celebrations, but said authorities would look into the overdoses.

"This whole incident [is] of great concern to us and there will be a high degree of scrutiny of the operators of this event," he said.

Extra police officers patrolled the festival on its final day in Sydney, where it continued until midnight on Sunday.

Greens leader Richard Di Natale said policy makers must stop being negligent and introduce pill testing.

"We must get to a point in this country where we acknowledge that what we're doing at the moment is not working, its not preventing people from taking drugs, far from it," Dr Di Natale said.

"What it does do is it encourages people to take harmful substances when there are interventions out there that could prevent it."

Police to talk to event organisers about drug use

Victoria Police said the introduction of on-site pill testing simply would not work.

Chief Commissioner Graeme Ashton said the best laboratories require days to determine what is in a pill, and it is not possible to do it quickly.

"You could have some quick testing that someone says some drug's OK when it's actually not, caused by the rushed drug testing," he said.

"So to do that, at those sort of events, to do it safely and quickly is not really a practical option."

Commissioner Ashton said police had concerns about drug use at the same event last year.

"This year ... we're still getting these drug issues through this event," he said.

"We'll be talking to the organisers and trying to make sure that we can encourage them as strongly as we can to make sure [they] are addressing these issues."

Commissioner Ashton added pill testing was "a bit of a myth" at large public events.

"It's actually a fairly complex process done properly. So you could have some quick testing, that someone says some drug's OK when it's actually not, caused by the sort of rush that would be necessary to do that testing [at a festival]."

The ABC has contacted the Electric Parade Music Festival organisers.