A spokeswoman for Perth Airport said a warning system had also lit up, and all passengers safely disembarked after the unscheduled landing.
While a Qantas spokeswoman said there were "no reports of any passengers affected", a St John Ambulance spokesman said paramedics were treating 75 people at the airport for smoke inhalation.
He said some might need to be taken to hospital for further treatment.
It is believed the pilots may have been affected.
"We're working closely with the Department of Health should a mass transport be required," the spokesman said.
Earlier, a Qantas A380 passenger jet travelling from Dubai to Sydney was diverted to Perth when the air conditioning developed a fault as the plane was over the Indian Ocean.
"As a precaution the aircraft descended to 10,000 feet and the captain requested a priority landing," Qantas said in a statement.
No one was injured.
In a statement late last night, Qantas said the technical fault on the QF7 flight to Dallas had impacted seat power, the in-flight entertainment system and some of the toilets.
The Qantas route between Sydney and Dallas is the world's longest non-stop route, taking 14 hours 50 minutes to the US and 15 hours 30 minutes on the return flight.
"While the aircraft could have continued flying safely to Dallas, the decision was made to return to Sydney in the interests of passenger comfort on what is a long flight," the statement said.
"Customers who don't live in Sydney have been put up in hotels for the night and all others given transport home. We've re-booked customers on another flight to Dallas leaving tomorrow morning."
- AAP
