The court unanimously upheld that the carrier had unlawfully laid off staff at 10 airports in 2020.
The ruling found that Qantas breached Australia's Fair Work Act, which protects employee rights.
Qantas said the outsourcing was a necessary financial measure.
The airline has faced public outrage in recent weeks, after reaping record profits despite a series of scandals related to its actions throughout the pandemic.
Qantas has also been accused of supporting a government block on the expansion of Qatar Airways flights to and from Australia - a move which critics say would have made the market more competitive and driven down airfares.
Last week, the airline's long-term boss Alan Joyce announced his departure from the airline two months earlier than scheduled amid the mounting controversies.
The Transport Workers Union - one of the main bodies representing workers which brought the case - said the finding was proof that "the entire Qantas board must be replaced by new directors including a worker representative".