Qantas could face a legal challenge over a condition in its controversial $2000 performance bonus scheme that strips a worker of their extra payment if they or one of their colleagues steps out of line and "harms" the airline.

The airline announced the $2000 cash performance bonus, plus a $500 travel voucher, for 27,000 frontline staff to much fanfare last month after handing down a record-high underlying profit of $1.6 billion.

ACTU secretary Sally McManus outside Qantas' headquarters on Monday. Credit:James Alcock

But Qantas was soon under fire after workers were told they would only receive the cash part of the bonus if and when they signed new wage agreements, which for some would not happen for another three years. Unions branded the bonus a "bribe" that would also make it harder for workers to reject any proposed enterprise bargaining agreements (EBAs).

Fairfax Media has confirmed a provision that workers will lose the bonus if they - or one of their colleagues - engage in any conduct that "harms" Qantas.