NSW Police were called to Bankstown Hospital about 3am on Sunday after a 26-year-old man died.
The man had attended Transmission Music Festival, a large indoor trance music rave hosting more than 18,000 people, at Sydney Showground on Saturday and arrived at the hospital about 1.30am.
Investigations into his death are underway and a report will be prepared for the coroner.
More than 130 people were treated by medical professionals at the festival, with some suffering issues relating to drug use and heat exhaustion.
At least 12 people were taken to hospital after drug use, with seven of those people requiring intubation.
Meanwhile, six men, aged between 19 and 23, have been charged with drug supply offences after police found them in possession of MDMA.
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A 23-year-old man was charged with drug supply after he allegedly had 253 MDMA tablets on him.
He was refused bail to face court.
A 21-year-old man was also charged with drug supply after he allegedly had 149 MDMA tablets on him.
He was granted bail to face court on Thursday.
The four other men were issued Court Attendance Notices and will face court at a later date.