The Australian Border Force is quietly slashing staff numbers at airports over the busy Christmas period and is believed to have suspended a fleet of boats supposed to protect the nation's northern waters, in cost-cutting moves that insiders say threaten national security.

The cuts to critical frontline operations, which critics have blamed on "catastrophic" budget blowouts, raise further questions about the performance of the quasi-military organisation following reports that it is plagued by a toxic culture of bullying and harassment.

Leaked emails from two regional commanders show the Border Force is taking dramatic action in the face of "significant budget pressures", including slashing casual staff by up to two-thirds in some areas. Sources say the organisation is becoming increasingly casualised, which suggests the reduction in overall workers will be substantial.

Leaked emails show the Australian Border Force is slashing casual staff at airports in the busy Christmas period. Credit:AAP

In one email to staff, acting regional commander for Victoria and Tasmania Greg Dowse says the entire Department of Home Affairs, including the Border Force, is implementing "significant measures" to ensure budgets are met, including "a reduction in the number of attendances that are offered to intermittent and irregular employees", or casual workers. Sources say the changes will mostly affect airport staff.