An imminent tightening of security at Parliament House threatens to deny public access to its signature sloping lawns, as authorities worry about the iconic building's ongoing vulnerability to a terrorist attack.

The restriction of access to the upper lawns - which has been pushed by security agencies in the past but never eventuated on heritage grounds - could alter the nature of the giant complex, which was designed to allow the people of Australia to walk over the heads of their elected representatives.

While details of the proposed upgrade are being tightly held, Fairfax Media understands a number of changes have already received the support of the major parties in the House of Representatives and will be progressed with senators this week, with a view to works beginning over summer.

New reinforcements are believed to include the installation of security barriers across areas currently accessible to the public, the erection of fencing and gates around the Senate and House of Representatives entries, fewer pedestrian entry points, and additional machine gun-equipped police patrolling its perimeters.