Sydney Airport passengers can expect long waits tonight with mass delays and 39 flights cancelled due to a sudden staff shortage.
Delays are expected to continue tonight after a number of air traffic controllers called in sick.
"We had a number of unexpected staff absences in the Sydney Tower today," an Airservices Australia spokeswoman said.
The ABC understands seven Qantas flights and 14 Jetstar flights have been cancelled.
Virgin has cancelled 14 flights, and Rex and Tiger Air have each cancelled two.
The airlines have been contacting affected travellers throughout the day via text and email.
Virgin Australia, which has also been notifying customers of delays, has been contacted for comment.
"Due to a shortage of Air Traffic Controllers, your flight to/from Sydney on 18 January may be delayed or cancelled," read one text from Qantas to a passenger.
Jetstar has also been contacting travellers, and the airline is offering vouchers as refunds for cancelled flights.
The airline is also letting customers organise to fly on alternative services.
Airservices Australia said the staff shortage was due to "a mix of illness and other personal reasons" and it was dealing with the issue.
"We have replacement staff on their way in and expect to improve capacity by early evening," the spokeswoman said.
"We do not anticipate any ongoing disruption to services over the weekend."
Sydney Airport said all affected flights were at the domestic terminal.
