Osteopath BJ Field said he had been shocked to hear of the plans last week, which would have a serious impact on the number of patients able to access his business, which in turn would devastate staff numbers and threaten the viability of the business, an employer of 28 people.

Plans to use the carpark as a construction compound, with the loss of 250 carparks, were revealed in the draft environmental impact statement for the tram.

Dr Field said about 40 per cent of patients at Canberra City Osteopathy and Physiotherapy required wheelchairs to walk or could only walk very short distances, including people with severe neck and spinal pain, pre and post-operative treatment including for knee and hip replacements, and pediatric patients. Walking to the Marcus Clark carparks or the Canberra Centre, as suggested by the government, was "simply unrealistic".

The business had invested more than $500,000 on fit-outs, including a lift, and the carpark was of paramount importance to its viability, he said.

Gary Rumble, of Kingsize Big and Tall, said he was "absolutely gobsmacked" that the government would "even consider such a ridiculous proposal". Many of the store's customers came from interstate and many also had health issues and needed to be able to park close by and safely, he said.

He pointed also to the plan to use the site for tram workers to park and be shuttled to other locations, an idea he said was "disgraceful and biased", with other businesses and workers in the city having to find and pay for their own parking. Mr Rumble promised a campaign to protest the proposal, describing it as dangerous, unfair and ill-considered.

Emilio Cataldo, of Cataldo's Salon, said the closure of the carpark would be the death knell of the Melbourne and Sydney buildings.

"They continually call them iconic buildings, they do a lot of lip service, but they don't really do anything to ensure the survivability and viability of these buildings," he said.

People would find alternative places to do business during the four-year closure and once new patterns had developed they would not return, he said.

It was not a level playing field between "the big end of town", which had convenient parking, and strip shopping, which didn't.

"Why would you take out a key carpark that's already struggling to serve the current businesses?" he said. "There doesn't have to be so much pain that you allow businesses to fold - you just make it so hard, you put up so many barriers for people to come in, and that's what happens."

Justin Kavanagh of Thirst said the eatery's customers wanted to park close by and feel safe out at night. The Melbourne building was already competing against new restaurants in the Canberra Centre, the Kingston Foreshore and other precincts with ample parking and would take another hit from the closure of the carpark.

She pointed also to the trucks, dust and construction equipment associated with a works compound which were "not a good idea near a heritage building", and would make the area still less attractive to visit.

Mezzalira owner Pasquale Trimboli said the business owners had been "treated for fools", presented with a done deal that would risk the Melbourne Building going the way of the Sydney Building and becoming derelict.

He questioned putting a construction compound "prime position, smack bang in the middle of the city right on the traffic lights in the main thoroughfares" to provide parking for tram workers while other city workers and businesses were pushed out. "Y