An outage at Sydney Airport air traffic control grounded flights to and from the country's largest airport during one of its busiest periods.
The systems were down from about 5.20am Monday, as thousands of passengers converged on the airport at the start of school holidays.
It took about three hours to resolve the software fault which caused flight delays and cancellations around Australia.
An Airservices Australia spokeswoman said the technical issues which plagued Sydney Airport had been resolved but flights would be delayed throughout the day.
"Airservices experienced a software issue affecting Sydney Airport's air traffic control and management system, resulting in major disruptions to flight operations this morning," she said.
"The software fault failed to convert from night-shift operations to day-shift operations, consequently one air traffic control console was operational for the morning peak when in normal circumstances six to eight consoles are operating.
"The safety of the travelling public is our first priority and traffic capacity was reduced to maintain safety standards."
There were chaotic scenes at Sydney Airport earlier in the morning as passengers on all airlines were told they would not be able to depart.
Passengers at Sydney were advised to board their planes but then had to wait on the tarmac. A number of flights were cancelled.
Arrivals were also affected with airline staff saying only 15 planes could land per hour. Under normal circumstances, a maximum of 80 aircraft arrivals and departures per operational hour is allowed.
A Sydney Airport spokeswoman said passengers should check with their airline to see if their flight is affected.
A Melbourne Airport spokesman confirmed that some flights were held back due to the outage at Sydney airport.
Passengers on a Virgin flight waiting to depart Sydney were told that flight plans were being submitted by fax before being processed manually by air traffic control staff.
Grounded passengers took to social media to express their frustration with the lack of information being provided about flights.
Finding out more from Tweets passengers already on planes are sending than what is been communicated at Sydney Airport right now.— Andrew Barker (@AndrewBarker21) September 24, 2017
Uh oh. Delays at Sydney Airport after a radar failure. Had to happen in Monday peak hour. pic.twitter.com/uEIYEMDwLk— Nadine Flood (@NadineFloodCPSU) September 24, 2017
The Sydney problem will have major knock-on effects for the whole country throughout the day.
"If one airport sneezes the others will catch the cold because we're all interconnected," a Brisbane Airport Corporation spokeswoman said.
She said knock-on delays were already being felt in Brisbane from the Sydney event.
"A lot of flights will be impacted because it is one of the busiest routes to and from Brisbane."
Melbourne Airport also confirmed likely delays throughout the day.
- With AAP