Home affairs won't be unchecked 'behemoth'

Claims a new home affairs department will be a "sinister" concentration of executive power have been rejected by the senior bureaucrat who will run the organisation.

The Turnbull government is establishing one mega-department that will oversee Australia's national security agencies including the Australian Federal Police and domestic spy agency ASIO, and immigration and border protection authorities.

"(Any) suggestion that the establishment of home affairs will somehow create an unchecked, extra-judicial apparatus of power is ill-informed ... fallacious, and unworthy," Immigration Department Department secretary Michael Pezzullo told a Senate hearing on Monday.

Mr Pezzullo hit back at characterisations of a "behemoth" department or a "sinister" concentration of executive functions which won't be able to be supervised.

"Both of these criticisms are wrong," he said.

The department would not be meddling in the running of independent agencies.

Existing arrangements for the oversight of ASIO, federal police, Australian Border Force, Australian Criminal Intelligence Commission and financial intelligence watchdog won't change, Mr Pezzullo said.