Australian carrier Qantas reported on Thursday a $5.5 billion plunge in revenue during the second half of 2020 and said international passenger flights would not resume until October as the pandemic continued to devastate the industry.

The country's biggest airline said it suffered an underlying pre-tax loss of Aus$1.03 billion ($814 million) in the six months to December 31, with statutory losses climbing to Aus$1.47 billion.

"These figures are stark, but they won't come as a surprise," said Qantas CEO Alan Joyce.

"A year ago, none of us knew just how big an impact Covid would have on the world, or on aviation. It's clearly worse than anyone expected," he said.

"Border closures meant we lost virtually 100 per cent of our international flying and 70 per cent of our domestic flying -- three-quarters of our revenue -- around Aus$7 billion -- went with it."

Joyce noted that the company had already seen revenue fall Aus$4 billion during the first half of 2020, bringing the total impact of the pandemic to Aus$11 billion.

"That is a massive number, probably a bigger number than any other company in Australia is experiencing because of Covid," he told a news conference.

Joyce pushed back the expected resumption of international passenger flights from July to the end of October, but said the cost of keeping those planes on the ground was largely being offset by increased freight operations.

With Australia's successful containment of the pandemic, Qantas flagged a return to 60 per cent of pre-Covid domestic capacity by the end of March and 80 per cent by the end of June.

Qantas had posted a $1.9-billion loss for the year ending June 30 as the coronavirus pandemic gripped the global economy.

Joyce said a total of 8,500 staff would lose their jobs due to the crisis, and another 7,500 would remain suspended until the resumption of international flights.

Around 100 planes have also been grounded as part of a Aus$10 billion cost-cutting blitz and restructuring effort that Qantas said would save it Aus$1 billion a year from 2023.