Speaking at Parliament House, Mr Morrison announced the number of government departments would be reduced in order to reduce congestion in the bureaucracy.

Four new departments would be created from February 1, 2020, he said.

The Department of Environment and Energy will be broken up, with the environment section to go into the Department of Agriculture, Water and Environment.

Energy will move to the new Department of Industry, Science and Resources.

The Department of Communications and Arts will be folded into the new Department of Infrastructure, Transport, Regional Development and Communications.

The Department of Education and Training will be merged with the Department of Employment, Skills, Small and Family Business to become the Department of Education Skills and Employment.

Services Australia, which was announced as the new name for the Department of Human Services in May, will also become a new executive agency in the Department of Social Services.

There will be no changes to ministerial portfolios as part of the reshuffle.

But five departmental secretaries are out the door.

This includes Department of Employment boss Kerri Hartland, Department of Human Services head Renee Leon, Department of Communications and the Arts boss Mike Mrdak, Department of Agriculture chief Daryl Quinlivan and Department of Industry head Dr Heather Smith.

David Fredericks from the Department of Environment and Energy will be the new secretary of the Department of Industry, Science, Energy and Resources.

Former Immigration Department boss Andrew Metcalfe will return to the ranks of the Australian Public Service to become secretary of the new Department of Agriculture, Water and Environment.

Mr Morrison said the government would respond to the Thodey Review, which was handed to the government in September, next week with further changes on the horizon.

When asked Mr Morrison said the changes had not been made as a cost-saving measure and decisions about jobs would be left to departmental secretaries.

"Department secretaries and others will undertake the normal things that they do in managing their budgets and those who were previously performing functions in the areas they were in other departments will now perform those functions in new departments.

"This isn't about any cost savings measures. I expect frankly all department secretaries to be realising maximum efficiencies for how they run their departments every day of the year, that's their job. That's why they're paid to do, what they do and I expect them to continue to do that.

And whatever decisions they take over the next 12 months, 2 years, 5 years, they'll take those decisions, they're not decisions the government is taking."

The Prime Minister's office told The Canberra Times decisions about the Average Staffing Level were up to departmental secretaries and didn't answer questions about how many people would be in each of the new departments.

Outgoing boss of the Department of Communications and Arts Mike Mrdak told his staff in a memo on Thursday he wasn't given the chance for any input in the decision.

"I was told of the government's decision to abolish the department late yesterday afternoon," Mr Mrdak said in the memo.

"We were not permitted any opportunity to provide advice on the machinery of government changes, nor were our views ever sought on any proposal to abolish the department or to changes to our structure and operations."

Mr Mrdak said Australia had the best public service in the world.

Outgoing secretary of Services Australia Renee Leon was more reserved, telling staff:

"The Prime Minister announced some changes to the structure of the public service that will affect our department. The department will become an agency in the Social Services from February next year.

"I am sorry to say that this also means I will no longer be your Secretary from then.

"I do not expect any change to the department's commitment to service delivery and to putting the customer at the centre - a commitment I know you all share."

Both Ms Leon and Mr Mrdak said staff would be updated about the changes in the coming weeks.

ACT Liberal Senator Zed Seselja said Mr Morrison had put his stamp on the public service.

"As Senator for the ACT I welcomed the Prime Minister's comments making it very clear these reforms are not a savings exercise - they are about ensuring taxpayers money is spent most effectively to deliver the services Australians expect."

Senator Seselja thanked the outgoing secretaries for their service.

"At a personal level I have known Mike Mrdak since he was a mentor of mine in the Department of Transport and Regional Services. I hold him in very high regard and I particularly want to thank him for his outstanding service to our nation."

Labor has called on the Prime Minister to guarantee there won't be further job losses

The Community and Public Sector Union has condemned the changes, claiming that creating 'super-departments' will undermine the effective delivery of government services.

"Moving buildings and merging departments does not fix the service crisis created by [the] government," CPSU National Secretary Melissa Donnelly said.

"The Prime Minister is wrong if he thinks slashing departments is going to improve services to the community. If Scott Morrison was serious about making sure Australians can rely on government services, he should stop gutting jobs and lift the [hiring] cap," she said.

The changes come amid claims that morale in key government departments such as Human Services is falling under pressure from heavy workloads and poor leadership.

A recent census of the public service found that less than 60 per cent of staff felt their agency cared about their health and wellbeing.

Ms Leon's departure has surprised many of those who have dealt with her.

Prominent economist Nicholas Gruen, who worked with Ms Leon while she was secretary of the Department of Employment, said she was one of the few senior leaders to understand the significance of human-centred design, which emphasises the importance of the human perspective in developing solutions to problems.

Dr Gruen, director of Lateral Economics consultancy, said in his dealings with Ms Leon she appeared to have a genuine understanding and interest in using the approach to improve the functioning of her department.

New Departments and bosses: