Passengers flying in and out of Sydney are being told to expect delays after the airport's air traffic control tower was forced to evacuate.
About 20 staff had to leave the tower soon after 11am on Friday after detecting smoke on the ground floor.
All outbound flights were temporarily grounded and those coming in to land had to either be diverted or maintain a holding pattern.
Air traffic controllers were cleared to return to the tower just after 12.30pm and operations returned to normal soon after.
Air Services Australia are processing arrivals, after temporarily grounding all flights operating in and out of Sydney Airport while authorities responded to the incident.
"There will be an ongoing impact on the network but we are trying to facilitate as best we can," Airservices Australia said in a statement.
"Passengers are advised to contact their airline for more details about the status of their flight."
Fire and Rescue NSW Inspector Bryce Jonas said the fire alarms were triggered by smoke on the tower's ground floor.
Crews used thermal-imaging detectors to find the source, which appeared to be a battery back-up that had malfunctioned.
A carbon dioxide extinguisher was used to cool it down, Inspector Jonas said.
"It could have been disastrous if it was a large fire to such critical infrastructure," he told AAP.
"They did the right thing by self-evacuating."
A number of passengers who were scheduled to fly out took to social media to say they were stuck on the tarmac because of the suspected fire.
Among those stranded were members of the Cronulla NRL team.
