"Unfortunately the three parties in Government have been unable to reach agreement."
Located 10km north of Waihi, any mine would enter from outside conservation land and tunnel underground to the minerals, which would be processed at Waihi.
OceanaGold planned to drill for several years to come, an investor statement by CEO Michael Holmes said, with first production in 2026.
By ruling out mining on the surface, the company has not had to apply to the Department of Conservation for access.
DoC's permissions manager, Judi Brennan, said DoC had received no application from the company and would only get involved if surface impacts were flagged.
"At this stage, and if they don't have any surface impact, we have very limited input based on the Crown Minerals Act [CMA]."
Minister of Energy and Resources, Labour's Dr Megan Woods, said the decision to approve the conversion of an existing exploration permit into a mining permit was made by MBIE following a determination that the relevant criteria under the CMA had been met.
"Whilst this permit has been approved, there remain a number of other regulatory processes, such as those under the Resource Management Act, that will consider wider factors such as environmental impacts.
"Work on a second tranche of changes to the Crown Minerals Act began this parliamentary term and will be pursued," she said.
OceanaGold general manager for Waihi, Bernie O'Leary, said there were technical, social, cultural and environmental studies, testing and evaluation to do before determining if the company could mine under conservation land at WKP.
"Any future applications to develop a mine would involve an extensive process of consultation and design to ensure the mine is developed responsibly. Our planning will include modern underground mining methods, with no processing or tailing storage facilities on conservation land and with minimal surface expression, for example, ventilation shafts."
Delahunty said the Government stated "there will be no new mines on DoC land" and allowing a permit at Wharekirauponga (WKP) - a stronghold for the endangered Archey's Frog - was a broken promise.
"Labour and the Greens have campaigned on this issue and promised action and we have had nothing but betrayal.
"This is an election issue and we would expect that Greens call out Labour on this issue and demand Labour protect DoC land in any coalition agreement if they are returned to Government," Ms Delahunty said.
"Now is their time to stand really strong on this ... I'm challenging them quite hard because ... it's unacceptable to walk away from a promise like this.
"There's been no progress on this and in the meantime the mining industry has gone hard."
She said many people believe Department of Conservation land cannot be mined and they will be very disappointed.
One of the world's most endangered frog species lives in the area where the mining permit has been issued, with OceanaGold having to relocate the rare Archey's frog under its agreement with DoC for surface activity at WKP.
Coromandel Watchdog had applied under the Official Information Act for a copy of OceanaGold's survey of the frog and other rare native species. A total of 73 Archey's frogs were found — 66 at the drilling and campsites and seven seen while the ecologists walked along informal walking tracks between the proposed drilling sites. As of August 2018, OceanaGold had drilled 8000 metres into the habitat.
Ms Delahunty said the Minister of Conservation could deny access to the area and the resource consents to mine could fail in court: "But we have lost all faith that the Government means to protect DoC land. Nevertheless we will fight this every step of the way."
Mr O'Leary said consultants and specialists would be brought in and extensive study of Archey's Frog would investigate what, if any, effect an underground mine could have and what mitigating actions may need to be implemented.
OceanaGold had explored for 15 years and announced a high-grade resource under the Coromandel Forest Park in February 2019.
Coromandel Green Party candidate Pamela Grealey said she is "gutted" by OceanaGold's plans to expand and to mine under DoC land.
"People have been kept out of areas at Wharekirauponga for several years now by private prospecting and Oceana's extension of mining there is a less-publicised element of the company's expansion."
