One man has died and two others are in a critical condition after taking an unidentified drug at a party at Mount Lindesay, on the Queensland and New South Wales border.
One of the survivors has been flown to the Gold Coast University Hospital for treatment, while the other has been taken by helicopter to the Toowoomba Base Hospital.
A Queensland Police Service spokesman said they were discovered after police were called to a party, where there were about 500 people, about 10:20am.
They said several people began to show an adverse reaction and required urgent medical assistance.
A Queensland Ambulance Service spokesman said they were originally called out for five patients but two men refused treatment and fled into the bush.
Police are searching for them and detectives are investigating the incident.