Rocks the size of houses went tumbling into the Martha Mine pit when about two million tonnes of rock collapsed from the north wall.

The slip happened just before 5am on Tuesday at the open cast mine in Waihi and was the second in 12 months, says OceanaGold senior community advisor Kit Wilson.

Around two million tonnes of rock fell in the most recent Martha Mine collapse. Photo: Christel Yardley/Fairfax NZ

Kit says the company has been monitoring rock movement with radar equipment for more than six months.

Small rock falls are expected until the slip stabilises.

'This slip is not unexpected,” says Kit.

'In fact, we advised the local community late last week that we believed there would be an event of this nature over the next few days.”

OceanaGold bought the mine from Newmont Gold in November 2015, seven months after the first slip in April.

No staff were in the pit at the time of the most recent rock fall and there was no risk to the public, he said.

Plans to re-enter the pit to continue works have to be modified and the company will proceed when it is safe to do so.

Green Party MP Catherine Delahunty​ says the slip put the future of the Martha Mine in doubt.

'It seems that OceanaGold are not managing it properly because it continues to slip. This is the second major slip on that north wall.

'Our concern is the people of this community have so much uncertainty being left with literally an unstable situation right at the heart of their town.”

Hauraki mayor John Tregidga batted away suggestions of an uncertain future in Waihi.

He says the mine has been out of operation since 2015 but work at the Correnso​ Underground Mine has continued.

'I accept the original design in the consent should have highlighted this particular fault but . . . is there any risk to the community? Absolutely not,” says John.

'All that is at risk is the companies operation and that's not a risk to the community at all.”

- Additional reporting Elton Rikihana Smallman/Stuff.co.nz