Alan Joyce, the chief executive of Qantas, said that discussions with Boeing had commenced some months ago and the changes to the B787 orders were appropriate in the current climate.
He denied the decision had been influenced by the announcement this week of a design issue and further delay to the aircraft's first flight.
Joyce said the "operating environment for the world's airlines has clearly changed dramatically" since Qantas announced its original B787 order in 2005.
"Delaying delivery, and reducing overall B787 capacity, is prudent, while still enabling Qantas and [budget subsidiary] Jetstar to take advantage of growth opportunities and market demands, both domestically and internationally."
|Qantas said last month that it expects to weather the global aviation crisis [EPA]
Last month Qantas said it expected to weather the aviation crisis - the industry's worst on record - without having to further cut capacity or jobs, or raise new capital.
It added that it retained the ability to buy up to 50 additional aircraft.
But the company has forecast a loss in the second half of its 2009 financial year, and announced in March it would shed 90 top management positions, adding to 1,500 job cuts announced last year.
Aviation analysts have warned that more Boeing customers could follow Qantas' move.
Boeing, already pummelled by the economic slowdown and Pentagon budget cuts, said earlier this week that it would delay the first test flight of the Dreamliner.
The Dreamliner is a carbon-composite plane that promises to pioneer an era of lighter, more fuel-efficient planes.
Customers with Dreamliner orders have expressed disappointment over the latest delay which was due to a structural flaw.