The peak public sector union has expressed serious doubts about a new national security super-department, warning the plan could prove disruptive, costly and result in less effective outcomes for the public.
The Turnbull government is expected to establish a new Canberra department bringing together ASIO, the Australian Federal Police and Australian Border Force, in a model based on the UK government's Home Office structure.
Cabinet will reportedly consider the plan on Tuesday with an announcement expected later in the week.
Immigration Minister Peter Dutton is likely to oversee the portfolio, with department boss Michael Pezzullo considered a possible pick for secretary.
The plan - considered the biggest shake-up in the structure of Australia's national security systems in decades - would also likely see widespread changes for public servants, as well as department and agencies bosses.
Community and Public Sector Union boss Nadine Flood said she held serious concerns about the effectiveness of the "significant public policy changes".
"Merging, splitting and moving agencies is disruptive and not a pre-requisite to them working better together," Ms Flood said.
"The merger of the Department of Immigration and Customs, which resulted in the creation of the Australian Border Force, has been very difficult and is still not fully bedded down after more than two years.
"Officers we represent would be concerned if we see an even larger restructure with significant challenges for the operations of these agencies."
ASIO has about 1800 staff, while Border Force includes about 5000 personnel and the Australian Federal Police is made up of about 6650 sworn officers and unsworn staff.
Detail about agency structure, location and workforce size are yet to be detailed.
Ms Flood, who represents Immigration staff and protective service officers within the AFP, said the model being considered could be unwise.
"There's a disturbing irony in quoting the UK as an example, given their Audit Office has raised serious concerns about the effectiveness of similar moves there," she said.
"We followed the UK in merging Immigration and Customs to create Border Force, now we could follow them again in putting Border Force into a Home Office super portfolio."
She warned against repeated agency restructures, funding cuts and falling staff morale.
Acting shadow attorney-general and shadow minister for national security Katy Gallagher slammed leaks ahead of cabinet endorsing the plan.
"We'll always listen to the advice of the security agencies and the experts first - and we'll always work in the national interest.
"I'm staggered that such critical changes to national security would be leaked from within the government so carelessly today," Senator Gallagher said.
She called on the Coalition to be extremely careful in any overhaul of Australia's security arrangements.
"Australians would be very concerned if a new national security super department is being created just to keep Peter Dutton happy, especially when it's so vehemently opposed by so many of his colleagues.
"We expect any changes to be based on the recommendations of our security agencies. We want to hear what the experts have to say," she said.