A generic diagram of a berm (3) and ditch (4). Credit:Wikimedia Commons

"There was a stage at which the berms were a serious contender for increasing external security. Then they came off the agenda. No one ever told me why they were dropped – they must have come up with some technical problem."

Berms are typically low in height and can run alongside a waterway such as a ditch or moat. They provide an effective barrier to vehicles, such as tanks or trucks, but in most cases are easily crossed by foot. The berm idea is said to have caused quite a stir among MPs who attended the briefings.

Senator Leyonhjelm described the proposal as "like a moat". He said it was discussed before the July election and was among a number of much more serious security proposals that could have severely restricted the public's access to Parliament House.

One of those was the construction of a "more substantial perimeter", he said, that would have installed a "big steel fence" about 100 metres from the building. Currently, any member of the public can drive around Parliament House or walk atop the building toward the roof.