About 5000 people attending the Genesis Black Subculture trance music event at the Hordern Pavilion were sent away when police closed down the event at 2am, about an hour before it was due to conclude.

A NSW Police spokeswoman said police from the Surry Hills Local Area Command and the dog unit conducted a drug operation when the dance party kicked off at 7pm.

Officers soon became concerned about the number of drug-affected patrons, as well as evidence that drugs were being supplied at the event, the spokeswoman said.

Police said that, during the night, paramedics treated four partygoers for suspected drug overdoses. The patients were taken to St Vincent's Hospital for further treatment.

The police spokeswoman said it was not clear what type of drugs the patients had allegedly taken.

About 2am, police returned to the Hordern Pavilion and issued the promoter and licensee with a short-term closure notice, and the dance party was closed down.

Police from the Surry Hills Local Area Command are continuing to investigate the incident.

Before the event, Genesis Events had posted a message on its Facebook page saying it had a zero tolerance-policy towards illicit substances at its events.

The company later posted that it was disappointed with the event's closure.

Hi Sydney, everyone is safe!We are as disappointed as you are with the early end to tonight's event but hope you can...Posted by Genesis Events on  Sunday, 4 October 2015