More than 90 blocks are set to go under the hammer on Thursday in the first mass auction of the year.

But concerns about the safety of the blocks have arisen, after loose-fill asbestos contaminated bricks were found on a cleared block in Griffith.

Two families, the Steves and the Jeffcotts, paid $1.6 million for the block and planned to build a duplex there.

However the construction of their dream home has been put on hold, after first bonded then loose-fill asbestos contaminated remnants of the old home were dug up by contractors preparing to lay the new slab.

Compounding matters, they cannot pursue the government over their mounting out-of-pocket costs because of a clause in their sale contact preventing them making a claim against the ACT government or the demolition contractors should any contamination remain.

ACT Law Society president Sarah Avery said she was concerned about families like the Steves signing "these extraordinarily restrictive contracts".

"We have previously publicly voiced our opposition to the form of these contracts. We also raised concerns with the ACT government last year that the indemnities in the contract were extremely one-sided and appeared to leave buyers with no recourse if remediation of their block of land was incomplete or not done properly," Ms Avery said.

"This is very harsh. In general, if you pay for something, you expect it to work. These contracts, in effect, say that it doesn't have to work. We were worried, that like with all human endeavours, something might go wrong, and that buyers would be left with no remedy."

Ms Avery warned prospective buyers to get a copy of the contract and legal advice before bidding.

"Buyers need to be properly informed before they spend their money and commit to a property. This goes for all land sales, but especially these ones where the stakes might be higher than buyers expect," Ms Avery said.

Velocity Conveyancing Group principal Andrew Satsias, who first raised this issue in May 2016, said the clause defied the purpose of the entire scheme.

"The very purpose of the government buyback was to eradicate the dangers of loose fill asbestos," Mr Satsias said.

"Documents of soil testing validation are available to buyers but provide no protection or assurances for them.

"As drafted, the contract for the purchase of a remedied Fluffy block absolves the government of all liability that the land has been remedied properly.

"The contract removes a buyer's ability to rely on the certificates of soil testing - this is contrary to the very reason why the Fluffy blocks were remedied in the first place."

The warnings come as an asbestos assessor who worked on the Griffith block calls for a new warning on certificates of sale for former Mr Fluffy blocks alerting buyers that the sites could contain buried rubble from the original asbestos-contaminated homes.

Ged Keane, from Keane Environmental, said builders should have an "unexpected finds procedure" in case they came across building rubble on the blocks, which could include asbestos sheeting.

The likelihood was greater in the homes that had been extended, given it had been common practice for builders to simply bury material that was not being reused in extensions.

While it was unlikely that new owners would find loose-fill asbestos, homes with wood frames and brick veneer had tended to show more hidden contamination than double brick because of the easier pathways for migration of loose asbestos from the ceiling into wall cavities.

In the Griffith case, Mr Keane said the original home had been extended a couple of times and it appeared building material had been buried on-site – away from the footprint of the house. He believed the contaminated brick was probably from the original house, rather than the extended home that had now been demolished.

His company had inspected the site after demolition and confirmed no visible asbestos material, and soil testing in the footprint of the demolished home had confirmed it was clear.

Mr Keane pointed to the first Fluffy block to be demolished amid fanfare, in Sternberg Crescent, Wanniassa, where he said asbestos hotspots had also been found in the soil as a result of pieces of bonded asbestos sheeting being used to stabilise underground sewerage pipes.

The company that knocked down the original house, AGH Demolitions, said they had not been alerted to any "hot spots", and would have returned to remediate them had one been found during the assessment.

However AGH Demolitions director Brett Gibson said they knew the house had been extended, because they had the original plans from the ACT government.

Mr Keane said buyers should be aware that if the Fluffy house had been extended at all, material could be buried on-site.

"I would update the sale advice to say that where houses have been extended there is a chance that original brickwork and construction material from the original houses might be buried on site, please adopt an unexpected finds procedure," he said. "That's what I would put on the certificate of sale going forward."

But Mr Keane said there was no reason for people to avoid buying the Fluffy blocks.

"If I had the money I would be buying them up all day long, I think they are a bargain."

In lieu of that, acting CFMEU secretary Jason O'Mara said buyers should try to get the original floor plans of the old house to see if it had been extended before demolition.

He said because the Fluffy blocks allowed a higher density of development, people were more likely to dig deeper down in creating basement garages and uncover potentially hazardous materials.

He suggested anyone buying a Mr Fluffy block should attend an asbestos training course, as building contractors are required to do before they step on site.

"If had a clearance certificate from an asbestos inspector, you wouldn't expect it to be there though," Mr O'Mara said.

"We spent $1 billion to fix this issue and if we missed some because it was a couple hundred millimetres from the top it's ludicrous."

The Canberra Liberals' planning spokesman Mark Parton said in a statement he was "gobsmacked" about the discovery.

"The families that bought this block have every right to feel gypped by the government," Mr Parton said.

"To now expect these families to pick up the tab for a job half-done is out of line."