The report, issued on Wednesday found the plane, a Boeing 737-838 with the registration VH-VXB, inadvertently flew outside controlled airspace at just 2000 feet above the land below, at an altitude of 6600 feet above sea level.

The report found the flight crew "overlooked" the altitude clearance limit of 7000 feet and "initiated changes in the aircraft's auto-flight system vertical mode", a setting that did not alert the crew when it steered the plane too low.

The crew had to be alerted by air traffic control of the breach, and then responded by moving the plane back to 7000 feet before continuing their approach to Canberra.

A spokeswoman for Qantas said that while the aircraft briefly descended below the limit, this was quickly corrected.

The report also blamed some of Qantas's procedures for the incident, saying these allowed the crew to turn off the mode that sets the plane's altitude, and forced the crew to rely on constant monitoring to make sure the flight was on track.

The bureau said this was not sufficient as it "removed the last automated safety system available to them to prevent descent at a height below what was allowed.

Since the incident, the bureau said Qantas had changed these procedures. Now altitude could be changed on the control panel only once the plane was within 4 kilometres of commencing its approach to an airport.

"As the ATSB report states, we have made a change to our procedures which means this type of incident cannot happen again," a Qantas spokeswoman said.

The report also warned of the dangers the auto-flight mode posed to pilots.

"Increased automation has enhanced situation awareness in some ways ... [but] it has undercut it in other ways by moving pilots from direct, continuous control to a role of managing and monitoring, to which humans are poorly suited," the report said.

Staff workloads were an issue in the accident, according to the bureau, which said the incident highlighted "the adverse effect of workload and task focus on flight crew performance".