If you had to imagine the worst possible occasion for an air traffic control system failure, Monday would have to be it.

Australia's busiest airport. Monday morning. Peak hour. At the start of school holidays.

With systems down at Sydney Airport, aircraft movements had virtually ground to a halt.

Broadcasts from Live ATC reveal how it sounded from the skies.

5:28am

Controller: Just for your information we're having issues with our flight plan and radar system. There's a possibility of delay.

Sorry, this audio has expired How it sounded from the skies: 5:28am

5:43am

Pilot (getting ready for take-off): Confirming we can expect [runway] 16 right for departure? Controller: Qantas 565 ah no, nothing at this stage. There's been a total radar failure within the Sydney basin, so it's a nationwide ground stop. The only arrivals that are being processed are those that are airborne, and I don't believe we're accepting any departures until the radar's back online. Pilot: The estimate of the delay? Controller: You got a piece of string? Pilot: Fair enough. Controller: The techs are working on it. Every time it reboots it locks up again so, ah yeah, until it's reliable there'll be no movements.

Sorry, this audio has expired How it sounded from the skies: 5:43am

6:10am

Controller: Emirates 412 [inaudible] tower G'day continue approach.

Controller: Emirates 412 I don't have a radar so I'm not sure who's behind you ... just standby.

Sorry, this audio has expired How it sounded from the skies: 6:10am

6:22am

Controller: Betcha never seen this place so quiet on a Monday morning.

Pilot: Like a ghost town.

Sorry, this audio has expired How it sounded from the skies: 6:22am

6:58am

Pilot: You guys going to get your [inaudible] together shortly, or are you gonna be out for a while? Controller: We're not sure, it's been out since five and we've got some sort of radar picture back and it's back as normal, not degraded, so it's not a good sign so not sure what's going to happen. Pilot: Good luck. Long day for you guys. Controller: Think we're gonna need it.

Sorry, this audio has expired How it sounded from the skies: 6:58am

Delays expected to continue through the day

Sorry, this video has expired Thousands of airline passengers are facing delays and some are not happy.

While the fault has been fixed, travellers have been warned to prepare for delays throughout as airlines deal with a backlog.

Passengers have been urged to contact their airlines for further information.

"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," Airservices Australia said in a statement on Monday afternoon.

"Airservices is confident that it was not a cyber incident, it was a hardware failure."

The ABC understands the fault was related to the flight planning system which feeds into the radar and without it, controllers cannot see which plane is which.

It is also believed the data connection between Sydney and Melbourne was down.