The giant steel fence slowly encircling Parliament House is a "monstrosity" and risks further alienating the public from their representatives, one of the few politicians to vote against the fortification says.

Construction workers placed the first 2.6-metre steel panels across the upper reaches of the building's sloping roof this week, with work to accelerate before MPs return to Canberra in mid-October. Large gum trees have also been chopped down, and manicured lawns ripped up to dig trenches for the structure.

The security fence is installed across the lawns of Parliament House in Canberra. Credit:Andrew Meares

The fence – and a series of new guardhouses – will eventually seal off vast stretches of the Parliament's exterior currently accessible to the public. The security overhaul will cost taxpayers $126 million.

Fairfax Media has spoken with a number of MPs who voted for the fence but are alarmed by the scale of the structure now they've seen it. None would criticise the project publicly because they still accept security took priority over the symbolism of broad public access to "the people's house".