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The approval is subject to an appeal process, which closes at the end of January.
Oceana Gold Waihi general manager Bernie O'Leary said if there were no appeals, work could begin in the new Martha underground mine by mid-2019.
''This is good for the company and good for Waihi. We now have ten years of work ahead of us,'' he said in a statement.
The new underground operation would give Oceana's 350 staff and contractors and the people of Waihi an assurance that the company was there for the long term, Mr O'Leary said.
Mining to date has been from the Correnso underground mine at Waihi.
Oceana's chief executive, Mick Wilkes, said the consent was a great outcome for the company, Waihi township and region, offering certainty for the next decade.
''Waihi has had a long, rich history of gold-mining that dates back to the 1800s, and since then over 11million ounces of gold have been produced in the Coromandel,'' he said.
Project Martha will also have mining return to the open pit, which had been closed since 2015 due to slip, including a second 2016 slip which was a 2million tonne rockfall.
There will also be some underground mining just south of the current pit under residential properties, Mr O'Leary said.
As with some previous underground operations, some of the proposed tunnelling would be under the Waihi township, a part of which sits at the edge of the open pit.
Since announcing an upgrade of the inferred gold estimated resource at Waihi in August, to a total 339,000oz, Oceana had recently completed another total 10km of test drilling at multiple points along two underground drives.
The week-long consent process was heard in November by three independent commissioners, who heard submissions from Oceana, the Hauraki District Council and Waikato Regional Council, Ngati Hako, the Royal Forest and Bird Protection Society of New Zealand, Fish & Game, NZ Transport Agency, and Waihi Community Forum, as well as individuals.