The federal government denies its focus on national security is slowly melting away.

Liberal senator Russell Trood is grilling senior bureaucrats including National Security Adviser Duncan Lewis in a Senate estimates hearing in Canberra on Tuesday.

"There has been a steady decline in the funding of national security options," Senator Trood told the hearing.

"It reflects a steady decline since prime minister (Kevin) Rudd was in office."

Mr Lewis, a former senior member of the SAS, denied the claim as did the government's senate leader Chris Evans.

"There is nothing about Mr Lewis that looks dissolved," Senator Evans said.

He suggested some functions of Mr Lewis and others within the Department of Prime Minister and Cabinet had been "picked up by other departments".

© AAP 2011