Opposition Planning Minister Alistair Coe said revelations that the Miowera rural property in the Tidbinbilla Valley has an historic asbestos dump in a gully, plus newly discovered ground contamination around buildings also called into question the accuracy of the ACT government's Asbestos Register.

He believed the site required a thorough investigation and wanted to see planned remediation of the area – as well as its associated costs.

A spokesman for ACT Health Minister Simon Corbell said on Friday "As the tenders are being assessed and due to the appropriate confidentiality arrangement, ACT Health is unable to provide any indication about what is involved in these works and at what cost."

ACT Work Safety Commissioner Mark McCabe is also waiting to see details of ACT Health's remediation plan, following a notifiable incident just before Christmas.

Contract workers were potentially exposed to asbestos fibres in November after digging up and breaking old piping near the access bridge. Work was halted and the pipe was recovered.

But workmen returned to the site to dig and sandbag soil later that month despite the express warnings of Ben Broers who has been the contracted farm manager at Miowera for eight years.

Soil the men were working in was subsequently found to contain asbestos and work was again halted. Mr Broers has since had his contract terminated – which the government said was due to it expiring.

In a separate incident, a workman was hired to mow tree lanes over ground that had exposed fragments of asbestos sheeting. Mr McCabe said this work should not have been allowed.

Mr Coe said these incidents called into question whether the indigenous drug and alcohol referral service should ever have been slated for Miowera.

"We believe there is merit in a rural facility for indigenous people, however this site has always been problematic – the neighbours who are the best-informed people when it comes to local issues knew the facility would be problematic, yet the government stubbornly went ahead despite local advice."

Dr Leo Shanahan, a prominent eye surgeon and 84-year-old former resident of the farm for 30 years said one of the reasons it harboured so much toxic waste was that a neighbouring family had close ties to the James Hardy asbestos company and received building materials for free – materials that eventually ended up in landfill in the Miowera gully. Waste was also transported there from when the Jolimont Centre was redeveloped in the 1980s, he said.

Mr Corbell's office confirmed that the directorate knew of the asbestos dump and had undertaken all due dilligence when it purchased the property in 2008. This dump would be removed as part of the development of the farm.

What was not known, however, was the extent of random asbestos sheeting fragments which have been identified all over the amenities area of the 320-hectare property.

The area had been "emu picked" by International Asbestos Removals in 2012 but "a range of reasons for finding additional pieces of asbestos containing material (include) potential erosion or stock movements," the spokesman said.

It was unclear at this stage whether this new contamination would be covered within the planned $11 million budget for the project which is still scheduled to begin within five months.

Mr Coe said the Opposition was keen to consider the costings for the project once they became public - as part of wider scrutiny of the way it had been approved.

"By taking this project out of the independent planning authority using his call-in powers, Mick Gentleman must take some political liability for the situation we now face," Mr Coe said.

Mr Gentleman is on leave – his office saying he would consider the issue when he returned next week.

Mr Coe also expressed concern about the circumstances in which Mr Broers lost his contract with ACT Health after eight years of managing the farm.

Mr Broers was outspoken with bureaucrats about his concerns workers spent four days working in contaminated soil. A spokesman for ACT Health Minister Simon Corbell categorically denied the moves was a response to Mr Broers' safety concerns and saying, the safety of workers and staff was of "paramount concern" to the ACT Government.

But Mr Coe said "in light of the thousand or so properties which require mitigation as part of the Mr Fluffy buy-back, the ACT Government should be encouraging a culture where people volunteer information about hazardous wastes. So if this contract termination has been in any way affected by Mr Broers being a whistle-blower, then it sends a terrible message and may lead to the establishment of a poor safety culture."

"I can't help wondering that if the shoe was on the other foot, and this was a private development - would the ACT Government have allowed this sort of progress to be made without the appropriate investigation and thorough mitigation?"