Pike River mine has a newly rebuilt seal at 170m deep, but there's not yet a final plan on what to do next.
The strategy for going beyond the seal needs to be finalised and put to WorkSafe.
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Recovery Agency chief operating officer Dinghy Pattinson told Newshub it will depend on tests of the atmosphere.
"We're putting together the final details of what that trial will look like over the next couple of weeks, then we'll start the trials."
Pattinson said everything depends on what is found during the testing phase.
"[WorkSafe] may come back with more questions, require more information at times and that's why we're working very closely with WorkSafe and their experts."
Overseas ventilation experts have been brought in for the operation.
A small team took part in a symbolic re-entry of Pike River Mine in late May, a few weeks after it was originally planned due to concerning oxygen readings from inside the mine.
Pike River Re-Entry Minister Andrew Little later told Newshub Nation at the time he believed the reading was due to faulty equipment.
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"It looks almost certain that there is an issue about the monitoring equipment and it is faulty, there is more testing to do," he said on May 4.
"What it now is that all of the monitoring equipment will now have to be tested, that will take some days possibly a couple of weeks."
Twenty-nine men were killed in a series of explosions inside the mine in November 2010. The previous National Government had said the mine was too dangerous to enter.
The current Labour Government disagreed and pledged during the 2017 election campaign to go back into the mine