One woman has died, another is in an induced coma and almost 70 revellers were charged with drug offences at the Stereosonic music festival at Sydney Olympic Park.
Police say a 25-year-old woman became seriously ill during the festival on Saturday and later died in Concord Hospital.
A 22-year-old woman has also been taken to Westmead Hospital in a critical condition and placed in an induced coma.
Meanwhile, 69 people were charged with possessing and supplying drugs, a number police described as "alarming and disappointing". A further 23 were issued with cannabis cautions.
More than 48,000 people flocked to the Stereosonic 2015 festival at Sydney Olympic Park yesterday.
A major police operation – involving officers from South West Metropolitan Region, the Dog Unit, Mounted Unit, Public Order and Riot Squad, Traffic and Highway Patrol, and Police Transport Command – was focused on the precinct to ensure the event was safe and secure.
The commander of the police operation, Assistant Commissioner Frank Mennilli, said police were disappointed at the behaviour of many of those who attended.
"The police operation is about minimising harm and providing a safe and secure environment for revellers to have a good time," Assistant Commissioner Mennilli said.
"This number is alarming and disappointing, especially after we warned revellers about the dangers of drug use.
The precinct went into "lockdown" earlier this week and regular "sweeps" were conducted by police and sniffer dogs to stop people from hiding drugs in the venue and picking them up once the festival was in full swing.
Two people were allegedly caught trying to hide drugs in the area, including one man who allegedly drilled a hole in a building to try to stash drugs there.
Police will prepare a report for the coroner on the death of a 25-year-old woman.
Almost 200 people were found in possession of prohibited drugs at last year's festival, when police said one man was lucky to be alive after he was found not breathing.
Footage also emerged after that December event showing a wild brawl in which a man was seen being punched in the head and knocked unconscious.