Two men in their 20s have died after attending the Knockout Outdoor music festival in Western Sydney.

NSW Health confirmed the men died in hospital after attending the event yesterday, while paramedics made nine urgent medical transfers from the festival to hospital.

At Listen Out festival in Centennial Park, held the same day, one person was transferred to hospital.

NSW Police were called to Sydney Olympic Park just before 1am, where paramedics were treating a 26-year-old man.

He was taken to Concord Hospital, where he died from a suspected drug overdose.

In a separate incident, a 21-year-old man was treated at a hotel on George Street in Chippendale, before he was transported to hospital where he also died.

Detective Superintendent Simon Glasser said police were waiting for autopsy reports before concluding how the men died, and would not confirm they were drug overdoses.

“This shows that people can die at music festivals,” he said.

“Drugs can have some horrible consequences. When you mix that with the heat of yesterday, it’s an added risk factor that can cause adverse affects.”