Police say any attempt to recover the bodies of the Pike River Coal miners is a long way off, with temperatures in the West Coast mine reaching about 600 degrees celsius.
They say efforts to prepare the mine for a recovery operation are continuing, but the situation remains extremely volatile.
Twenty-nine men died after a series of explosions at the mine near Greymouth which began on 19 November.
A GAG machine used to neutralise gases in the mine has been operating. Although flames are no longer visible at the mine vent, oxygen is still getting in, meaning a risk of more explosions.
Another danger with capping the vent has also been identified - it could cause the machine to stall, which is likely to cause an explosion.
But while gases continue to escape through the vent, it is too dangerous for helicopters to fly overhead to install the cap.
The next stage is to pump nitrogen into the mine in an effort to lower the temperature.
There is one machine capable of doing this in New Zealand and another in Australia. Each is being assessed to see if it can be used at the site.