SYDNEY—Qantas Airways Ltd. will seek to terminate contract terms with long-haul flight attendants, the carrier said on Thursday, in a move that would bring large pay cuts, after union members voted against more flexible rostering. Qantas said it was the first time in its history it had sought to terminate an employment terms agreement, or a contract between employers and unions, which is a rare and heated move in industrial relations in Australia. The move sets up the airline for a high-profile union battle at a time when it also needs to strike agreements with short-haul crew in order to seal the business case for a major Airbus SE narrowbody aircraft order. If it succeeds, cabin crew pay and conditions would revert to a much lower Australian minimum industry standard while a new pact is negotiated. Qantas’ latest contract offer to long-haul flights attendants was rejected by 97 percent in …