Qantas Airways has cancelled 28 flights and delayed numerous others because of a union strike.

All of the affected flights are domestic services, said a spokesman. Australia's flag carrier has put in place a contingency plan, where it will operate larger aircraft such as Boeing 737s and 767s on domestic routes, he added.

The cancellations and 27 delays were due to a strike by ground staff who are members of the Transport Workers Union (TWU).

"The TWU are asking for 15% pay increase over the next three years, which is just not sustainable in the current economic climate and when these employees are already the highest paid in the Australian aviation industry," said Olivia Wirth, Qantas Group's executive for government and corporate affairs, in a statement.

"The union is also trying to place restrictions on Qantas, which would remove our flexibility to scale up or scale down our workforce in line with busy and quiet periods."

She added that the TWU is allowing its members to be paid 12% less at Qantas rival Virgin Australia and allow them to have a lower pay scale for new starters.

"But they are demanding that Qantas does not have the same flexibility," she added. "We have been in negotiations with the TWU since May and are willing to offer reasonable pay increases. We encourage the TWU to remain at the negotiating table instead of going on strike."

However, the union said that it has done all it can to "avoid service disruptions" and said that Qantas representatives did not "seem interested in settling anything" during their meetings.

"TWU members overwhelmingly supported taking industrial action in ballots conducted by the Australian Electoral Commission over the last month throughout Australia," it added in a statement.

"More than 95% of members supported taking action - a damning reflection of current Qantas employees' frustration with executive management," the TWU said.

The Australian Licenced Aircraft Engineers Association and Australian and International Pilots Association (AIPA) has also had disputes with the airline in recent weeks and Qantas says that the three unions are "working together to attack" the carrier in a "coordinated campaign".

"We have been in negotiations with the TWU since May and are willing to offer reasonable pay increases. We encourage the TWU to remain at the negotiating table instead of going on strike."

The TWU said its members will return to work on Wednesday and the Qantas spokesman said the airline does not expect any further disruptions.

Source: Air Transport Intelligence news