Machines being used to make the Pike River coal mine safe are failing to neutralise the atmosphere inside.
Twenty nine men are dead following a series of explosions which began on 19 November.
A 'gag' machine and a machine which pumps nitrogen into the mine are now working around the clock .
The 'gag' machine has been pumping water vapour into the shaft for three weeks.
Police say that overseas, the machine usually neutralises mines in about four days.
Further evidence of the problems being faced by the recovery operation occurred overnight on Sunday when the 'gag' machine was shut down for repairs and gas levels jumped sharply, prompting the evacuation of the site.
In another disappointment, another machine brought over from Australia to push pure nitrogen into the mine is proving too weak to fight the noxious fumes inside the mine.
Canterbury University engineering geologist David Bell has been inspecting the mine and says the outlook is not good.
Professor Bell says the situation is by no means under control and it could be up to two months before the mine is safe to enter.
Superintendent Gary Knowles told Morning Report on Tuesday that Professor Bell had "pretty much" summed it up.
He conceded there is a real chance the mine might have to be sealed if the machines fail.
He said the problems at Pike River are because of the mine's unique structure and geology.