Police have hinted that the ongoing labor row at Qantas as the possible motive behind the death threats that the air carrier's chief executive, Alan Joyce, received at his residence.

"It's coming soon Paddy. You can't even see it," the Daily Telegraph quoted part of the letter, which also said that the unions will fight Joyce.

"Qantas is our airline, started and staffed by Australians, not foreign filth like you," the letter said. Joyce was born in Ireland.

Federal and New South Wales police advised the Qantas boss to upgrade his home security. As a result of the death threat, Joyce hired the services of Close Personal Protection, which provided him bodyguards to accompany him to some events.

Joyce also wrote a memo to all 35,000 employees to be alert to threats since it is not only him but also some senior staff who have been threatened. Some of them suffered smashed car windows and damaged homes after they refused to join the series of strikes that hit Qantas over the past two weeks.

Some also got handwritten letters calling them scabs and warned of consequences on the Qantas managers and their families if they would perform the tasks of workers who would join future job walk offs.

Qantas union leaders, however, denied any involvement in the death threats.

"We have put out instruction to members to be cautious about what they say and do. There is no place for intimidation in the workplace," Australian Licensed Aircraft Engineers Association head Steve Purvinas told The Courier-Mail.

He added that it is instead Qantas maintenance managers who are bullying union members to cut corners on maintenance.

"Unions stand for non-violence, freedom of association and safety. This is not the behaviour of a unionist," Transport Workers Union (TWU) National Airlines official Darren Brown was quoted by the Sydney Morning Herald.

Brown added there is no evidence of involvement of any TWU members in the threats and expressed hope the police probe will lead to the swift prosecution of the offender.

Besides the engineers, pilots, baggage handlers and ground crew have struck over pay dispute and work conditions. Another industrial action is slated on Friday initiated by the TWU.

Qantas had warned air travelers to expect delays in their flights.

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