Sydney Airport has come to a standstill after smoke was detected inside the air control tower.
All air controllers were evacuated around 11.40am and firefighters are on scene.
An Air Services Australia spokeswoman told news.com.au a full ground stop was in place.
"No aircraft are departing or arriving at this time. We will update you as we know more."
Fire and Rescue NSW said 20 people needed to be evacuated from the building.
Airlines are in the process of notifying its passengers as hundreds more sit on the tarmac and wait for their planes to take off.
Jeremy Scott was one passenger still stuck on a plane.
"When you're on the tarmac @SydneyAirport about to take off for holidays and the pilot announces a fire in the control tower, everyone's been evacuated and airport in lockdown #badvariance," Mr Scott wrote on Twitter.
In an update, Mr Scott said the pilot had again spoken to his hundreds of passengers and admitted he "hadn't heard from the control tower".
Status screens in Sydney Airport are also informing waiting passengers there are delays.
"Flight delayed due to Sydney Tower evacuation," the screens read.
Flight maps showing where planes are near Sydney showed dozens of aircraft sitting and waiting on the tarmac.