Workers at the giant department were told on Monday that a new round of redundancies was targeting big reductions in its executive and senior executive ranks.

The news is the latest shock to a workforce that has been slashed by at least 3000, according to the latest available figures, since 2012.

Bosses at the department will not say how many of the thousands of EL and SES public servants it wants to cut but departmental secretary Dennis Richardson says he wants to cut middle managers who manage few or no workers.

Unions have already condemned the move, saying the "indiscriminate" redundancy program was "bone-headed" and would result in the loss of vital military specialists, many of who were employed at EL1 and EL2 level, but were not supposed to manage anyone.

But Defence wants to improve its ratio of managers to rank-and-file workers which came under fire from the recent "First Principles" review which found there were nearly 2000 managers at the EL2 level, typically supervising a staff of three or less.

There were another 4000 or so staff members at the next level down, EL1, who typically supervise only one or two employees despite public service guidelines calling for middle managers to be supervising a staff of five to eight.

Mr Richardson, in a message sent to staff on Monday afternoon, said the department was still looking for "stability" in its headcount, despite having lost 17 per cent of its staff in recent years.

"As part of the Department's downsizing, there is a need to reduce EL numbers beyond that delivered through the natural attrition process over the past three years," the secretary wrote.

"Following the VR process for SES and EL officers, it is likely that the Department will be close to reaching a point of some stability in overall numbers."

Mr Richardson said the department might be in a position to begin hiring again in mid-2016.

But technical union Professionals Australia, which has many members working in Defence science and technology jobs, lashed out at the decision, saying it was likely to further erode Defence capability.

Professionals Australia director, Dave Smith, said he was frustrated with the lack of detail, including the refusal to confirm the number of jobs to be targeted, in Defence's announcement.

"The decision by the Government to have an agency wide, indiscriminate redundancy program for EL employees is rash and bone-headed," Mr Smith said.

"It borders on contempt for the Senate inquiry and will have consequences for critical projects and research programmes.

"It is driven by a general view that subject matter expertise should be no higher than an APS6 despite the need for world class and nation leading science, engineering and national security expertise and for that expertise to be valued accordingly.

"In engineering fields this will make Defence uncompetitive for the skills they need.

"Einstein would struggle to get a job in this organisation."