Asbestos taskforce head Karen Doran confirmed the find at 32 Phillip Avenue, Downer, as demolitions passed the halfway mark with 502 Fluffy homes now demolished.

The discovery brings the total number of Fluffy-contaminated homes to 1023.

Twenty-six owners have refused to take part in the government's buyback and demolition, which leaves 996 homes being demolished, 12 of them privately by their owners. Another 11 townhouses or units attached to Fluffy houses, described as "impacted" properties, are also being demolished.

Ms Doran said the Phillip Avenue house, discovered by a tradesperson repairing the home in October, had been missed in the survey of homes built before 1980 when the Commonwealth went on a hunt for properties containing the loose asbestos insulation in their ceilings. The survey led to the ceilings being cleaned from 1988 and 1992.

In 2011, a separate house in Bradfield St, Downer, that had been missed in the clean-up was discovered. The demolition of that property in 2013 revealed the extent of contamination and sparked the ACT government's decision to demolish all of the homes.

The houses that were missed in the clean-up are a much bigger risk to residents, since the insulation has remained in the ceilings for the past 25 years. In some cases, including a recently demolished home in Duffy, residents have removed the asbestos insulation themselves without knowing the material was asbestos.

Ms Doran would not discuss details of the new Downer house and said the history was unclear, but the asbestos discovered was remnant, rather than the bulk of the original insulation. "There is evidence that there has been asbestos there at some point," she said.

The family was considering whether to join the buyback.

Ms Doran said there was no reason to think Downer was any different to other homes, despite the discovery of two missed houses in the suburb. There were no plans for a more detailed survey of Downer houses, she said, but she urged anyone concerned about the possibility of Fluffy insulation in their ceilings to have it tested. The cost of the test would be reimbursed if the test came back positive, she said.

The Downer home was the first to be identified since the buyback was announced in October 2014.

The fact that a tradesperson had recognised it showed the compulsory training for building workers was effective, she said, urging anyone concerned about the possibility of Fluffy insulation in their ceilings to have it tested. The cost of the test would be reimbursed if the test came back positive, she said.

The updated demolition program plans another 30 demolitions this year, and 358 in 2017, most in the early or middle part of the year. That will bring the total to 890, and leaving a little more than 100 to follow. Owners are allowed to stay in thier homes until mid 2020 under the program, so the final demolitions could happen much more slowly.

A number of suburbs have been cleared of Fluffy homes and the government is reselling the land in the hopes of recouping some of the costs of the $1 billion buyback and demolition.

Ms Doran said the demolition schedule was subject to industry capacity, weather and other issues But by late next year, only about 130 properties would remain - those where owners wanted to stay longer, and more complex cases. She envisaged a "natural break" in the demolition program in 2018 until the final homes were demolished in 2019-2020.