Three people critically ill
A Victorian woman, 21, and Sydney man, 23, died after collapsing at the event at Penrith on Saturday.
Three people, including a 19-year-old man, are critically ill in hospital while 700 of the 30,000 attendees were treated by medical staff, police said.
"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," the premier told reporters on Sunday.
"This is an unsafe event and I'll be doing everything I can to make sure it never happens again."
Police urge people to take responsibility
Police said a Jamisontown woman, 26, remains in a critical condition at Nepean hospital while the 19-year-old man from Artarmon was flown to Westmead Hospital and is in intensive care.
A Newcastle man is also critically ill while more than a dozen people were treated at Nepean hospital for drug-related issues.
Toxicology reports are expected within a week to uncover exactly what drugs, if any, were taken by those who fell ill.
Acting Assistant Commissioner Allan Sicard said 180 police were at the event and had worked with organisers to try to keep revellers safe.
"What we can't do is be in people's heads, be in people's decision-making processes when they decide to takeillicit drugs," he told reporters.
Business as usual is killing people: Greens
Greens MP David Shoebridge said the police presence at the festival was aggressive and called for festivals to introduce pill-testing, amnesty bins and other harm minimisation measures.
"We can't keep repeating past mistakes," he said.
Mr Shoebridge said the police strategy so far had not worked and needed to change.
"Business as usual is killing people," he said.
Organisers reiterate zero tolerance approach
Defqon.1 organisers, having dealt with multiple deaths in the event's ten-year history, 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.
Police say 10 people were charged with drug supply offences, including two 17-year-old girls who allegedly carried 120 capsules "internally" into the venue.
Ecstasy, cocaine and GHB were among the drugs confiscated from 69 people at the festival held at the Sydney International Regatta Centre.
Local detectives have formed a new police strikeforce, dubbed Highworth, to investigate the two deaths.