Australian carrier Qantas has decided to offload its ageing Boeing 737-400 planes, but the airline insisted Monday it was part of an overall fleet modernisation and not a "garage sale".

News that 21 of its Boeing 737-400s would be on the market from this year comes less than a week after the airline said capacity and jobs would be slashed as it grapples with natural disasters and rising fuel costs.

"This has been part of our fleet strategy all along," a Qantas spokesman told AFP. "It's not like we are having a garage sale."

He said the sales, which would continue over the next few years, would allow Qantas to replace the 737-400s, known as the workhorse of the skies, with the more modern and efficient 737-800s.

Qantas would not speculate on how much the sales would earn, but reports said each plane could fetch as much as Aus$20 million ($20.8 million) thanks to interest from low-cost Indian and Southeast Asian carriers.

Sales would boost cash flow for Qantas, which last month said the combination of recent natural disasters and rocketing fuel prices had created the airline's "most serious challenge" since the global downturn.

Announcing the cuts, Qantas chief executive Alan Joyce said there had "never been a time when the world faced so many natural disasters", with earthquakes in Japan and New Zealand, and Australia devastated by floods and cyclones.

Record floods in Australia's tourism destination Queensland prompted $82.5 million in cancellations and other costs while New Zealand's deadly earthquake wiped $15 million from the bottom line.

Japan's massive 9.0 magnitude quake and tsunami brought another $45 million in losses.

News of the Qantas sale comes two days after United States carrier Southwest Airlines cancelled hundreds of flights to inspect 79 of its Boeing 737 aircraft after a hole in a fuselage forced an emergency landing.