Residents fear the CSIRO's former headquarters on Limestone Avenue are being "wilfully neglected" and exposed to looting as a decision about their heritage value drags on.
The question of whether to demolish or heritage list the buildings has hung over the site, near Campbell High School, since it was bought by developer Doma Group for $20 million in 2016.
Eric Martin, from the National Trust, said vandalism and looting of the buildings had been rampant for some time, but the trust believed the site had heritage value, both for its architecture and the endangered flora and fauna on Mount Ainslie.
Visiting the site on Friday, he said damage appeared to have increased since his previous visit a month before.
"It's just wanton neglect," Mr Martin said. "There's broken windows, what looks like tracks from people driving around.
"It's quite clear that the fence is still not secure. There's a gap around the back so there's easy access in."
Residents told The Canberra Times they were concerned about "organised looting", after an access ladder had been spotted hanging from an upstairs window.
One local said people had been pillaging the decommissioned office block for copper piping for quite some time but a "recent rash" of graffiti and looting during the past fortnight had many in the area concerned. He also reported seeing school-age children wandering around the site.
Police have been called out to the decommissioned block 17 times this year, though only twice since June. One incident in June was still before the courts, a spokeswoman for ACT Policing confirmed.
Doma has applied to the National Capital Authority to demolish the office block and put up a mix of town houses and apartments, but its proposal has not yet been approved.
In March, the Department of Environment and Energy was asked to issue an emergency heritage listing for the site. The department confirmed it was still undertaking an assessment of the request.
"While this process is being undertaken, there seems to be a total ignorance to the protection of the site," Mr Martin said.
"Now, the heritage values that did exist are being destroyed."
Residents said cars, including large four-wheel drives, were now regularly spotted "carving up" areas of the block known to be home to endangered plants and insects.
A spokeswoman for the National Capital Authority said there was no statutory timeframe for a decision to be handed down on the developer's works approval application.
"Normally four to six weeks would be what's reasonable for a works approval decision, including department consultations but unlike in the territory there's no statutory period for this," Mr Martin said.
"Still, regardless of where the development application sits, the developer should be protecting [the building]."
Doma director Jure Domazet said security measures, including fences and hired guards, had already been in place for some time at the site to deter looting, but had not been updated.
Doma had also removed some of the copper sought by thieves from the buildings.
Many locals have voiced strong opposition to the development and want to see the site preserved for its scientific history, having been home to the CSIRO for more than 40 years.
An independent report by GML Heritage, commissioned by Doma, said the redevelopment needed to consider the "identified heritage significance of the site".
On Monday, Mr Martin wrote to the developer on behalf of the National Trust, as well as the relevant government departments, to voice concerns about the building's condition.
Mr Domazet blamed publicity around the office block for an increase in looting and vandalism there.