Two young revellers have tragically died after collapsing at a Sydney music festival from suspected drug overdoses and another two are critically ill.

Emergency services responded to multiple reports of suspected drug overdoses at the Defqon1 music festival, which marked its 10th anniversary overnight, in Sydney's west yesterday.

The annual event - featuring hardcore techno, house and trance music, and dubbed by organisers as "the world's largest harder styles festival" - attracted about 30,000 revellers to the Sydney International Regatta Centre in Penrith. But just a few hours into the night, tragedy struck.

A 21-year-old Sydney man and 23-year-old Melbourne woman who both collapsed around 9pm were transported from the venue to Nepean Hospital but died soon after.

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A 19-year-old man from Artarmon was flown from the festival to Westmead Hospital in a critical condition. He remains in intensive care. A fourth reveller — a 26-year-old woman from Jamisontown — also remains in hospital in a critical condition.

One witness, who spoke on the condition of anonymity, told news.com.au that five revellers who became critical first presented to medical staff in a "semi conscious state".

"They just looked like people who were a little bit too drunk but then things went down hill very quickly," the source said.

Police said 13 festival-goers attended Nepean Hospital overnight for treatment for drug-related issues, while as many as 700 people sought assistance from medical staff on-site.

The Department of Health is fast tracking toxicology investigations to determine whether the casualties died after takingthe same drug, the Daily Telegraph reported.

NSW Premier Gladys Berejiklian on Sunday vowed to permanently shut down the "unsafe" festival.

"I'm absolutely aghast at what's occurred, I don't want any family to go through the tragedy that some families are waking up to this morning, it's just horrible to think about," Ms Berejiklian said.

"This is an unsafe event and I'll be doing everything I can to make sure it never happens again."

Defqon1 Australia organisers this morning released a statement to news.com.au in response to the deaths.

"The organisers of Defqon.1 Australia are deeply saddened by the tragic passing of two of their patrons at Nepean Hospital after attending the festival last night and would like to convey their sincerest condolences to their families and friends," the statement read.

"Thoughts and prayers are also with the young man and woman who are still in a critical condition.

"We are disappointed at the number of reported drug related incidents, we have a zero-tolerance policy in relation to drug use at the festival."

Festival organisers are working closely and cooperating with the authorities regarding the fatalities and the number of medical presentations made during the evening, a full investigation is currently underway, the statement continued.

"As this is a matter with the NSW Police and the coroner and out of respect for the families and friends we are not going to speculate on the cause of death and we will not be making any further statements or comments," it said.

A total of 10 people, including two 17-year-old girls who allegedly carried 120 capsules internally into the venue, have been charged with drug supply offences; three are due in court today.

A range of illicit drugs were seized including MDMA, cocaine and ecstasy.

In a statement, police said a total of 355 drug searches were conducted with 69 people found to be in possession of drugs, including the 10 charged with supply offences.

"A multifaceted operation was undertaken at the festival with resources including the Nepean PAC, Police Transport Command, North West Metropolitan Region Enforcement Squad, NWM OSG, and the Police Dog Unit," the statement read.

Defqon.1 organisers had warned ticket holders the festival's drug policy was zero tolerance.

"This means that all types of soft- and hard drugs are prohibited. If drugs are found, you will be handed over to the police," a statement on the festival's website said.

Strike Force Highworth has been formed to investigate the deaths of the two young people and an investigation is underway.
TWO young revellers have tragically died after collapsing at a Sydney music festival from suspected drug overdoses and another two are critically ill.

Emergency services responded to multiple reports of suspected drug overdoses at the Defqon1 music festival, which marked its 10th anniversary overnight, in Sydney's west yesterday.

The annual event - featuring hardcore techno, house and trance music, and dubbed by organisers as "the world's largest harder styles festival" - attracted about 30,000 revellers to the Sydney International Regatta Centre in Penrith. But just a few hours into the night, tragedy struck.

A 21-year-old Sydney man and 23-year-old Melbourne woman who both collapsed around 9pm were transported from the venue to Nepean Hospital but died soon after.

A 19-year-old man from Artarmon was flown from the festival to Westmead Hospital in a critical condition. He remains in intensive care. A fourth reveller — a 26-year-old woman from Jamisontown — also remains in hospital in a critical condition.

One witness, who spoke on the condition of anonymity, told news.com.au that five revellers who became critical first presented to medical staff in a "semi conscious state".

"They just looked like people who were a little bit too drunk but then things went down hill very quickly," the source said.

Police said 13 festival-goers attended Nepean Hospital overnight for treatment for drug-related issues, while as many as 700 people sought assistance from medical staff on-site.

The Department of Health is fast tracking toxicology investigations to determine whether the casualties died after takingthe same drug, the Daily Telegraph reported.

In 2015 Nigel Pauljevic, 26, was found unconscious in a tent at the festival and later died. A 23-year-old person died at the festival two years prior to that.

Scores of social media users have flocked online to advocate for pill testing at music festivals.

But the NSW Premier said the government will not consider introducing pill testing because the state has a "zero tolerance" to drugs.

"Anyone who advocates pill testing is giving the green light to drugs, that is absolutely unacceptable, there is no such thing as a safe drug," Ms Berejiklian said.

"I want to send a strong message to every young person ... You should not take drugs at these events or anywhere else, and last nights tragic consequences demonstrated this."