In a newsletter to Waihi township residents, Oceana said its consent conditions to operate the mine meant it had to leave the walls of the open pit in a "safe and stable'' condition, which at present was "clearly not the case''.
"We cannot leave the north wall as it is.
"Irrespective of whether we find a way to safely re-enter the open pit and resume mining or whether it is determined this will not be possible, the wall must be left safe and stable,'' Oceana said.
The slip, which came on top of a smaller slip a year earlier which has closed the pit since then, happened because of the effects of historic underground mining beneath the north wall, combined with the rock structure of the wall, Oceana said.
Close monitoring with sensitive equipment meant Oceana had been able to publicly predict the likelihood of the second slip days before it gave way, and this week it said there was still more unstable material which would fall.
"There are certainly some very large boulders within the slip area that will fall,'' the company said.
Monitoring of the pit's other walls showed the movement was restricted to the section of the north wall which had fallen.
Oceana said it would take months rather than weeks to determine the best course of action and the rockfall on the north wall would be "staying there for quite a while yet''.
"We will keep the community informed as we work through the issues and develop strategies and time frames,'' the company said.
The effects of the slip were limited to the north wall of the Martha open pit, and mining operations at Oceana's neighbouring underground Correnso mine had not been affected, and would continue as usual.
The first slip in April 2015 prompted Oceana to lay off about 50 contractors with Macmahon, but as no pit mining had resumed since then, there would be no job losses because of the latest slip, Oceana said.
