An estimated two million tonnes of rock and rubble have collapsed from the north wall of Waihi's Martha open pit gold mine - further delaying plans to re-enter the mine.

Oceanagold senior community advisor Kit Wilson said the slip, which happened just before 5am today, was not unexpected.

Mr Wilson said he watched some of the north wall collapsing and some of the rocks which came tumbling down were the size of houses.

"It was a pretty spectacular," he told the Bay of Plenty Times last night.

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"Members of the public may have heard the rock falling, but there was no danger to them at any time."

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.

"Last week we advised the community that we believed there would be an event of this nature over the next few days, possibly on Anzac Day, including through Facebook posts, but we were one day out, " he said.

Mr Wilson said following the first slip the mining contractors roles were terminated and 50 staff were let go, and the large majority had since found alternative work.

The company had been working towards re-opening the pit knowing 75,000 ounces of gold worth about $75 million had been left behind after the first slip, he said.

The rock fall was in an area that had been extensively monitored and the radar system could detect "very small movements".

"We have employed a range of monitoring systems to analyse the rock movement since we took over the mine six months ago, which has also allowed us to make precise measurements of the rock face and anticipate this sort of event."

Mr Wilson said 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.

"While this most recent event was not unexpected, it will require us to modify our plans for re-entry into the open pit. 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," he said.