The toxic Tui mine site is no longer at risk of collapse following work to stabilise the tailings dam.
Last year the Government announced a $16.2 million cleanup of Te Aroha's abandoned mine, regarded as one of New Zealand's most polluted sites
The project, a joint initiative between the Government, Waikato regional and Matamata-Piako district councils, involves strengthening the tailings dam and adding a new clay cap to prevent leachate polluting nearby streams.
Cement and lime was injected into the old tailings to stabilise it while the tailings were also reshaped and given an even, well-drained slope.
Regional council chairman Peter Buckley said having the old mine tailings stabilised was "a hugely significant milestone".
"It means there is no longer any risk of the dam made up of old tailings collapsing and sending toxic chemicals down into the Tui Stream and on to nearby flood plains."
Old mine tailings had been dumped at the site behind a geo-technically unstable dam built of the same tailings and local material. The tailings blocked a tributary of the Tui Stream and leached heavy metals into the stream.
Regional council river and catchment services group manager Scott Fowlds said the old Tui mine was Waikato's most hazardous site because of the range of environmental risks it posed.
Mr Fowlds said only reconstruction of the front face of the dam had stopped it failing previously.
It was estimated a dam failure could potentially spill 25,000-80,000 cubic metres of contaminants over four hectares. A cleanup of that would cost up to $168m.
Mr Fowlds said the old tailings dam was somewhat "out of sight, out of mind" but could have collapsed in an earthquake or storm event.
Regional council project manager Ghassan Basheer said empty cyanide drums left in the tailings had been treated within the stabilised tailings mound and were now well buried. A cap of clean fill is due to be placed over the newly shaped land by March next year.
The first stage of landscaping and planting of the site is set to be finished by the end of May.