As the decision drags on, the empty buildings have become increasingly dilapidated - and the target of looters and vandals.

Concerning footage has also emerged of hooded teens breaking into abandoned buildings around Canberra, including the old CSIRO site, their feet dangling over the edge of rooftops or balancing on walkways. Some videos feature close escapes from authorities or "overnight challenges" where trespassers try to outsmart security on site.

A spokeswoman for the Department of Environment and Energy confirmed last Monday the decision to heritage list the CSIRO site was still under assessment and would take "as long as it takes" but there was no normal timeframe for such decisions.

Property developer Doma Group owns both the CSIRO site and the abandoned Alexander and Albemarle towers in Woden, which have also become a playground for young "urban explorers" in recent years.

Doma, which pays for security at the sites, plans to knock down the office blocks to make way for apartments and townhouses.

But Eric Martin of the National Trust said Doma's own report of the CSIRO site found it had heritage value, both for its architecture and the endangered flora and fauna on Mount Ainslie. While the question of a listing hung over the site, those values were being destroyed by vandalism and looting, he said.

"The longer it drags on, it just frustrates everybody, it's not good for the company, it's not good for the community," Mr Martin said.

"A [decision] doesn't normally take this long...especially given...the proposal is there, the reports are there, it should be a relatively simple process.

"These decisions needs to be made so people understand where the situation stands."

Doma managing director Jure Domazet said he was waiting on the heritage process, which was quite complex, and he did not know when to expect a decision.

But, as the site continues to sit empty, some residents have also raised concerns about "organised looting" and "school age children" at the old buildings.

Similar problems have plagued the Woden towers, and Department of Health staff in the building across have long complained of people trespassing at the site, smashing windows, or else "jumping from roof to roof".

Several expressed fears "someone might be killed" after teens were spotted on the roof of the buildings last year.

A spokesman for ACT Policing said officers were concerned about all criminal activity but "by nature, identifying offenders in relation to abandoned buildings is very difficult".

In the past year, he said police had attended the former CSIRO site 18 times, including over trespassing offences, and the Alexander and Albermarle buildings in Woden at least four times.

"[Those] statistics do not capture pro-active police patrols where no offences or offenders were identified," the spokesman said.

While the government's lease variation charge made redeveloping the Woden blocks as offices unviable, Doma is currently preparing a new development application to turn the towers into apartments instead.