The prosecutors and other staff at the Office of the Commonwealth Director of Public Prosecutions have been given little detail about the potential outsourcing, but their union says the plan could see the most sensitive and vital court cases being handled by staff from labour-hire agencies.

The office confirmed on Monday that it was considering hiring some "junior prosecutors" through "third party providers" in an effort to manage the numbers of public servants on its books.

The Community and Public Sector Union says the CDPP, like other federal agencies and departments, is being pressured by the Turnbull government to slash its numbers of public servants.

The CPSU says an unknown number of prosecutors will be hired through contracting companies, with support staff also to be employed through similar third party arrangements.

The union says it met on Friday with CDPP management to raise concerns about the proposal on Friday, and wrote to Attorney-General George Brandis and Finance Minister Mathias Cormann seeking more information, but details remain sketchy.

CPSU Deputy Secretary Melissa Donnelly was scathing on Monday about the CDPP's plans.

"This is a terrible idea and will be concerning to all Australians," the union official said.

"Federal prosecutors in the CDPP do critical work in pursuing evil crimes such as terrorism, child exploitation, human trafficking, money laundering and tax and welfare fraud.

"This is a crazy proposal, it doesn't save the CDPP money.

"In fact if anything the opposite will be the case as the labour hire firm seeks to turn a profit; this proposal is just about meeting the Turnbull government's objective of cutting public sector staff.

"CDPP is being forced to reduce the number of staff it employs directly because of a cap on staffing levels imposed by the Government."

"We are concerned that outsourcing these roles would not only undermine the important work that federal prosecutors do but also damage the reputation of the CDPP as an independent prosecution service.

"Their work is too important to be outsourced to a private company."

"By driving the CDPP to such extraordinary steps, the Turnbull Government is taking a pointless risk with the important work and the hard-earned reputation of the office."

In a statement supplied to Fairfax, the CDPP said it was not currently using labour hire firms to supply prosecutors.

"The CDPP does not currently employ any federal prosecutors via labour hire companies," the statement read.

"In order to appropriately manage its ASL (average staffing level) cap, the CDPP may consider engaging some junior federal prosecutors via third party arrangements.

"In this context, other options being actively explored are seconding lawyers from other federal agencies to undertake work with the CDPP.

"The CDPP employs 308 practicing lawyers working across all jurisdictions and levels of court, with 74 new federal prosecutors engaged in 2016-17.

"None of those prosecutors were engaged via labour hire arrangements. "