The NSW Police have, without explanation, dropped their investigation into alleged death threats made against Qantas's chief executive Alan Joyce and other senior managers.

Complaints from Qantas about threatening letters and emails led to police forming a taskforce during the escalating industrial dispute with unions representing long-haul pilots, aircraft engineers and ground crew including baggage handlers.

But NSW Police have confirmed that Strike Force Barrine – made up of officers from Botany Bay Local Area Command – has decided not to continue the investigation.

A police spokesman would not elaborate on the reasons that the investigation had ended.

"As far as the investigation is concerned, it was alive and now it's not," he said.