More than 150 domestic and international flights were delayed out of Sydney Airport from 7am on Saturday, the second straight day of disruption caused by a shortage of air traffic controllers at Airservices Australia.

The Commonwealth owned corporation that runs Australia's air traffic control imposed flow management measures, spacing out departures to match the number of controllers it could put on shift. More than 100 services were delayed or cancelled on Friday. Thirty-three departures were held on Thursday morning.

Airservices Australia said the restrictions followed roster constraints, that it was working with airlines to limit the impact, and that it was expanding its training pipeline to add controllers and give its rosters more depth.

Richard de Crespigny, a former airline pilot, set out the mechanics of the restriction on Saturday. "There's a shortage of air traffic controllers and they're doubling the separation between aircraft to halve the flow which will cause delays of up to an hour all day today out of Sydney," he said.

That arithmetic is the whole story. Separation between aircraft is a safety minimum, and when there are not enough controllers to watch the traffic, the minimum goes up and the number of aircraft that can move goes down. Nothing failed. The system did exactly what it is designed to do when it is short of people.

Airlines for Australia and New Zealand, the lobby group for Qantas, Virgin Australia, Jetstar, Rex and Air New Zealand, said the repeated disruption was avoidable and that it had sought an urgent update from Airservices. "When there are delays out of Sydney, it impacts the entire national and international network," chief executive Stephen Beckett said.

Federal Transport Minister Catherine King said Airservices needed to do better at managing disruption, and that her office had asked the agency for an explanation and a pathway to improve service delivery. She said 95 new air traffic controllers were expected this year.

Recruitment figures and roster figures are not the same number. A controller counted in this year's intake is not yet a controller available for a Saturday morning shift in the Sydney tower, and the agency has not said how many of the 95 will be working that tower or when.

Airservices has no competitor. Airlines cannot buy the service from anyone else, passengers cannot route around it, and there is no market signal that arrives when the roster is thin. The minister and the agency are the entire accountability path, which is what three days of delays have been testing.

Neither Airservices nor the minister's office has given a date by which the Sydney roster will be fully staffed. Flow restrictions remain the agency's stated tool for managing the shortfall, so the next thin roster produces the same result: departures spaced, capacity halved, and passengers told at the gate.