NSW Police were called to Sydney Olympic Park, where the Knockout Outdoor Festival was being held, shortly before 1am on Sunday morning.
A 26-year-old man was treated at the scene by paramedics, before being taken to nearby Concord Hospital, where he later died.
In a separate incident, a 21-year-old man was treated by paramedics at a hotel on George Street in Chippendale.
He was transported to a hospital and died shortly after.
NSW Health has confirmed both men had attended a music festival before their deaths.
The cause of death for both men has not been disclosed.
NSW Police have reported that over 70 people were charged with drug offences at Knockout and Listen Out, which was held on Saturday at Centennial Park.
Police ran high-visibility operations involving licensing police, the Dog Unit and other specialist commands at both festivals.
Among the 27,500 people who attended Listen Out, 85 were detected in possession of drugs.
Of these, 37 field court attendance notices were issued, 29 criminal infringement notices and 19 cannabis cautions were given.
At Knockout, 27 people were charged with drug offences of the 53,000 who went to the Homebush festival.
Four people were charged with supplying a prohibited drug in an indictable quantity.
