A major shake-up of the federal public service has raised concerns the Morrison government's focus on environmental and climate issues will be downgraded as the areas are subsumed into two new mega departments.

The number of government departments will shrink from 18 to 14, four new departments will be created and five department heads will lose their jobs in changes that will take effect on February 1 next year.

Scott Morison has announced a major shake-up of the public service. Credit:Alex Ellinghausen

"Having fewer departments will allow us to bust bureaucratic congestion, improve decision-making and ultimately deliver better services for the Australian people," Mr Morrison said.

The energy and environment portfolios, which are currently housed together in one department, will be split.