A 23-year-old man has died at the DefQon.1 music festival in Penrith.
Police say they will prepare a report for the Coroner after the death.
84 other patrons were arrested for drug offences at the Penrith event overnight.
More than 18,000 people attended the dance festival held at the Sydney International Regatta Centre yesterday and police from Penrith Local Area Command conducted a drug dog operation.
Police believe that the 23-year-old Victorian man had travelled by car with several friends to attend the festival.
Shortly before midday, the man was taken to the venue medical centre. His condition quickly deteriorated and he suffered several seizures after which an ambulance was summoned.
He was then conveyed to Nepean Hospital where he had to be resuscitated after numerous prolonged cardiac arrests, however shortly after 10.30pm he died.
During the dance party police arrested 87 people with three of those detained for public order offences. The remainder were all arrested for drug offences.
Officers believe that 14 other patrons were conveyed to hospital after suffering apparent drug overdoses.
Police from Penrith LAC will prepare a report for the Coroner outlining the full circumstances of the man’s death.
Police are urging anyone with information about this incident to call Crime Stoppers on 1800 333 000.