After months of controversy and debate, new security fences are now being installed on the grassy slopes of Parliament House.

The home of Australian democracy was designed to allow visitors to walk over the top of their politicians at work, but this was reconsidered due to security concerns.

Visitors will be met with a 2.5-metre-high metal fence installed about three-quarters of the way up the lawns, which will stop people reaching the roof.

Leaving aside the changes, the symbolism and cosmetics — which have been questioned by the Australian Institute of Architects — the fence is one part of a $126.7 million security upgrade.

The merits of the fence had long been debated, but installation began on Tuesday morning. ( AAP: Lukas Coch )

The fence was formally approved on the same day a group of pro-refugee protestors abseiled down the front of Parliament House, unfurling a banner.

Prime Minister Malcolm Turnbull said the security improvements, which passed the Upper House with a clear majority, came "in an environment of heightened security".

"We have always got to make sure the people's house… is as open and accessible as it can be and we try to get the right balance there," he said.

Sorry, this video has expired In December 2016, hundreds rolled down the lawn of Parliament House for one last time

When the fence was first announced, the soon-to-be Home Affairs Minister Peter Dutton defended it, saying politicians needed to trust the advice from security authorities.

But others were not so convinced. Victorian senator Derryn Hinch said putting fences of the green slopes would be like "putting barbed wire on the Sydney Opera House".

Finance Minister Mathias Cormann said he did not feel vulnerable in the building and three Labor senators raised concerns about measures in the party room late last year.

Tougher security measures were introduced at Parliament House in 2014 after intelligence agencies picked up talk of potential threats to the building and government figures.