One Waihi resident who witnessed a large rock fall at Martha open pit gold mine described the sound as like a series of bombs going off and is now concerned more rocks may come down.

An estimated two million tonnes of rock and rubble collapsed from the north wall of the Waihi mine early on Tuesday further delaying plans to re-enter the mine.

Oceanagold senior community advisor Kit Wilson said the slip was not unexpected.

Jennifer Horgan, a social worker at Waihi Resource Centre, said she was returning from her walk along the fenced-off pit rim about 7.30am on Tuesday when she saw rocks coming down.

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"I was almost back at the pump house and saw the mine rescue team standing there watching the slip. I was just about to leave when I heard an almighty explosion.

"I thought a bomb had gone off." Ms Horgan said what followed sounded like a series of explosions that "went on for ages".

"It was an amazing sight. I was gobsmacked. Where the slip started was right where the old pit rim walkway had been diverted and fenced off a few weeks ago because of the potential risk of the wall collapsing. It's an area I have walked many times," she said.

Ms Horgan said her main concern was if there was a further major slip it may mean the walkway was further diverted limiting viewing access to the pit rim.

Mr Wilson said the company had been monitoring the north wall's movement using sophisticated radar equipment since Oceanagold bought the mine on November 1 last year, and following a smaller slip in April last year.

Since the April 2015 slip there had been only limited work in the open pit, all of it well away from any potential rockfall area.

"Our plan was always to resume work in the mine if this could be done safely.

"That plan has not changed, but it may take a while longer now," he said.

Hauraki Mayor John Tregidga said at this stage it was too early to predict what impact this failure would have on future mining within the pit.

"But the council will be working with the company on options to remediate the failure and explore what this may mean for the company's future mining in the pit," he said

Green Party MP Catherine Delahunty said residents who had contacted her were concerned about the legacy for the town if the pit kept collapsing.