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The council, in a statement this afternoon, announced it had issued "notices of desire'' under the Public Works Act, as part of a push to purchase the land and prevent any mining taking place.
Dunedin Mayor Aaron Hawkins said Foulden Maar was rich in biodiversity, essential for conservation and will allow scientists to better understand the history of life, southern hemisphere biogeography and climate change.
"We know that the Maar is considered by scientists to contain New Zealand's richest trove of fossils, including plants, insects, spiders, fish and pollen,'' he said.
"For these reasons, its important we act to preserve this very special place and prevent any mining taking place.''
Issuing the notices meant a process to bring the land back into public ownership would start, Mr Hawkins said.
The notices have been served on the owner and registered on the titles, and the council now had to obtain a valuation and negotiate "in good faith'' with the owner, he said.
If the parties could not agree on a price, one would be determined by the Land Valuation Tribunal, he said.
If no agreement could be reached, the council would have one year to decide whether to proceed to a compulsory purchase.