The ACT has more heritage locations deemed "at risk" than any state or territory.
The National Trust is unhappy with Canberra's apparent willingness to maintain a heritage facade while simply gutting or demolishing the rear of significant places.
The organisation says it is deeply concerned about the level of "internal destruction" planned for the Law Courts Building, which is to be gutted.
The trust says other ACT areas at risk are the housing precincts of Ainslie, Blandfordia 5 (a section of Griffith) and Reid, where there are "demolitions contrary to mandatory requirements ... leaving only facades of listed houses".
The organisation is also concerned about plans to infill a section of Lake Burley Griffin to make way for the Australia Forum and a possible new Canberra Stadium.
The ACT has three of the 10 sites in Australia listed by the National Trust to be most at risk.
In Victoria sites include the Argus Building in Melbourne and the Castlemaine Gaol.
The whole of the Murray Darling Basin is nominated as a region of natural, historic and Indigenous values at risk due to delays in producing an effective management plan.
ACT Chief Justice Terence Higgins and his judges have been lobbying intensively for years to replace the crumbling Supreme Court building on Knowles Place in Civic with a complex capable of meeting the modern needs of the court.
The ACT Government plans to gut the building and replace it with a more modern design, adding another storey to the two storey structure.
Two levels will be built underground for parking, along with subterranean courtrooms and a tunnel between the Supreme Court and Magistrates Court.
Attorney-General Simon Corbell says the only parts of the 1963 Supreme Court building to remain would be its facade, columns and coat of arms.
The interior would be gutted, except for timber panelling originally gifted by the states which would be reused in some form.
Mr Corbell said last year, when announcing the project, the building was not heritage-listed, but it was important to keep its design features in the new development which would confirm it as the central heart of the justice precinct.
Dr Peter Dowling, national heritage officer, said on Friday the National Trust was disappointed the nomination of the law courts building to the ACT heritage register had not proceeded.
"We are also extremely disappointed and deeply concerned about the level of internal destruction," he said.