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The diatomite of the crater lake has preserved a fossil treasure trove and a climate record covering 100,000 years from 23 million years ago.
Foulden Maar was to be mined by Plaman Resources, but the foreign-owned company went into receivership in 2019.
The council first signalled its intent to buy the 42 hectares owned by the company under the Public Works Act in November that year.
The Act gives the council a year to follow through.
But a year has long since passed and the council refuses to discuss the matter publicly.
Plaman Resources paid more than $600,000 for the 42 hectares of land and more than $5 million for the mining company associated with the permits to take the diatomite.
The 42 hectare area covered by the mining permit is surrounded by 430 hectares of farmland.
Before going into receivership, Plaman Resources had applied to the Overseas Investment Office to buy the farm so it could mine the diatomite reserves under it.
The maar is unique for New Zealand and stands at an incredibly preserved record of the environment and ecology over a 120,000-year period from 23 million years ago.
Plaman had intended to use the fossil rich diatomite as an animal food supplement for pigs, chickens and turkeys.
That plan drew the ire of many, including former Prime Minister Helen Clark, which led to the opposition to Plaman's purchase of the surrounding farmland and ultimately its demise.
A report went to Dunedin city councillors in October, but it was not public and was discussed behind closed doors.
A request by RNZ for the report under the Local Government Official Information and Meetings Act was refused by the council.
In response the council said withholding the information was necessary to maintain legal professional privilege, to enable the local authority to carry on without prejudice or disadvantage, and allow negotiation including commercial and industrial negotiations.
Questions to the council this week about whether its valuer had valued the site, when October's report would be made public, and whether the council was still pursuing the site under the Public Works Act were met with a single line response.
"Dunedin City Council has received a valuation of the Foulden Maar site and is in discussions with the receiver regarding the valuation. We are not able to comment further at this time," a council spokesperson said via email.
When asked again if the council intended on making the October update public and whether the Public Works Act declaration remained in effect, the council said it was unable to comment further.
Strath Taieri Community Board chair Barry Williams said the council needed to finalise its plans and make them public.
"I just wish they'd hurry up," he said.
"Seriously I think it's dragging on too long. It's pretty unfair for the landowners [of the neighbouring farm]."
He would raise it with council staff at next week's community board meeting, as he had done on several occasions in the past, but the community board had been told nothing about the process to date, he said.
"As far as I know no one knows anything. It's going to be raised in my chairman's report. I don't expect much of an answer, but it's just dragging on a bit - isn't it."