Mr Turnbull unveiled on Tuesday his long-awaited plans for the ministry (based on Britain's Home Office), which will be headed by Immigration Minister Peter Dutton and bring together ASIO, the Australian Federal Police, the Australian Border Force and other agencies.

Crucially, the attorney-general will continue to sign ASIO warrants under the new arrangements, allaying fears that protections for Australians under the rule of law could be eroded.

The major security agencies will also remain statutory bodies with legislation that ensures their independence.

Mr Turnbull said the announcement was the "result of years of planning and research", though there has been no formal review that has guided the proposal – the most significant aspect of which is that it merges domestic national security with border security.

Stressing the current system was working but improvements could be made, he said that "we need these reforms not because the system is broken but because our security environment is evolving quickly.

"It is becoming more complex, it is likely to remain so for the foreseeable future. We need a better structure to meet the challenge of the times. And that is why we're adopting a model which is closer to the British Home Office than the large-scale American Homeland Security Department," he said.

ASIO, the AFP and Border Force will all answer to Mr Dutton, but he will be supported by current Justice Minister Michael Keenan and a second junior minister who handles immigration.

A 2015 government review into counter-terrorism arrangements dismissed a US-style "super-agency" but acknowledged a small, flexible co-ordinating ministry could provide leadership.

Mr Turnbull said the planned arrangement would be small enough to provide that role.

A separate review of the intelligence community by former Foreign Affairs head Michael L'Estrange, released on Tuesday, did not recommend a home affairs ministry, though Mr Turnbull said this had not been the review's job.

That review did recommend the creation of an Office of National Intelligence which would co-ordinate foreign intelligence gathering and would be a souped-up version of the current Office of National Assessments, which largely analyses foreign intelligence.

Mr Turnbull said the government would adopt that recommendation as well as strengthening the independence of the Australian Signals Directorate – the Defence Department's cyber spies – by making the organisation a separate statutory body.

Expert responses to the announcement varied from critical to cautiously optimistic but generally did not signal any great benefits to the home affairs arrangement.

Allan Gyngell, a former head of the Office of National Assessments and now a professor at the Australian National University, said a first assessment suggested "several positive dimensions including better co-ordination of foreign intelligence activities through an Office of National Intelligence and improvements to the way in which warrants are issued".

Regarding whether bringing agencies under the home affairs portfolio would improve co-ordination and policy direction on national security, Professor Gyngell said: "I sort of buy that. I don't think it's going to be transformative but it's a plausible way of addressing co-ordination gaps and so on."

Dennis Richardson, the recently retired head of the Defence Department who has previously headed ASIO, said merging Immigration with the domestic intelligence and security agencies "could give some marginal improvements in relation to what information immigration [officials] might bring".

He said ASIO should not be hindered by answering to the home affairs minister while still having warrants signed by the attorney-general.

Australian Strategic Policy Institute head Peter Jennings said that beyond terrorism, more co-ordination was needed in the areas of cyber-security, critical infrastructure and protection against foreign influence.

"There's a lot of work to do to get organisational details sorted," he said. "I think the government has to very quickly set out in a discussion paper the sort of strategic picture they have in mind for the home office. That's not really done with one press release."

Michael Wesley, an international security expert at ANU said: "So my question is: Where is the evidence that this would be an actual improvement?"

Nick O'Brien, a former counter-terrorism police chief now at Charles Sturt University, said: "I think we've been extraordinarily well served by our police and security agencies ... Why would you change a system that's worked so well?"

The changes are to be finalised by June 30, 2018 – subject to approval of the national security committee of cabinet – with Mr Dutton to work with Senator Brandis in bedding down the changes.