Recovery workers at the Pike River Coal mine are still hopeful of starting a machine overnight on Wednesday designed to make the West Coast mine safe to enter.
Twenty-nine mine workers were killed following a series explosions which began on 19 November at the mine near Greymouth.
A fire burning underground in the mine has spread to the coal seam.
The GAG machine, a modified jet engine, will be used to shoot nitrogen into the mine to diffuse explosive gases and make it safe to enter the area where it is believed the bodies are.
The officer in charge of the operation, Superintendent Gary Knowles, says the recovery team is still waiting for the conditions to be exactly right before using the machine.
Mr Knowles is hoping to start the machine for a few hours overnight on Wednesday in an attempt to smother the fire before nitrogen is pumped in to neutralise gases in the mine and make it safe to enter.
He says it is important that no one's safety is compromised by starting it when the mine's atmosphere is dangerous.
Australian expert joins operation
Police hope an expert on underground explosions from Australia will give them a better picture of what to expect from the fire.
Australian scientist Dave Cliff, of Queensland University, is considered a world expert in underground fires.
Superintendent Knowles says having Mr Cliff on site is an integral part of the installation and operation of the GAG machine.
"He'll give us a better first-hand understanding of what we can expect to find and as the temperature changes, or anything changes in that mine, he can say 'this is what you're looking at'".
Mr Knowles says the scientist has been briefed daily about the challenges facing the recovery team.
Police are also liaising with forensic dentists, DNA specialists and doctors to get a better understanding of what may be encountered by recovery teams.
The recovery operation will continue on Thursday despite most of Greymouth stopping for a remembrance service at Omoto racecourse.
Prime Minister John Key is is calling on all New Zealanders to observe a two-minute silence at 2pm, at the time the service is due to start.
Thousands of people are expected to attend.