The police officer in charge of the operation at the Pike River Coal mine where 29 miners have been trapped for five days says the likelihood of a successful rescue is diminishing.
Superintendent Gary Knowles, speaking at a media conference in the West Coast town of Greymouth on Tuesday evening, says the situation is bleak.
Police Commissioner Howard Broad, who arrived in Greymouth on Tuesday with Police Minister Judith Collins, agreed with the assessment, saying the situation is looking bleaker by the hour.
At the media conference, CCTV footage was shown of the mine entrance when the explosion occurred on Friday afternoon.
The footage, taken from a camera outside the mine entrance, shows a cloud of dust blasting from the mine for about 50 seconds
Mine officials say the footage demonstrates the force of the explosion, which is likely to have occurred about two kilometres into the mine.
Grey District Mayor Tony Kokshoorn says families of the missing miners were subdued after watching footage of the blast. Mr Kokshoorn says the mood at the meeting was different from this morning.
Pike River Coal chief executive Peter Whittall told the news conference a camera was sent down on Tuesday to a fresh air base where workers can seek refuge in the case of fire. Mr Whittall says it showed some minor damage to the area, but no sign of any people.
Police say a rescue crew cannot go into the mine at present because the risk of secondary explosion is real.
Meanwhile, a second Defence Force robot has arrived by helicopter at the Pike River Coal mine to replace the one that broke down inside the mine.
Earlier in the morning, the first robot failed after coming into contact with water just 550 metres into the mine.
The Grey District mayor, Tony Kokshoorn, has appealed to the Australian prime minister, Julia Gillard, to have a robot sent from Western Australia as fast as possible for use in the Pike River mine.
The ABC reports an Air New Zealand plane is on its way to Western Australia to collect the robot, which is owned by the Water Corporation.
Mr Kokshoorn told the ABC the robot is the only thing that can tell relatives whether their loved ones are alive or dead.
The robot, owned by the state's Water Corporation, is controlled by a fibre-optic cable and has a range of up to six kilometres. It is equipped with cameras, lights and communications and gas-testing equipment.
Rescue continues on several fronts
Efforts are under way on several fronts to try to rescue the men, but police say the longer the operation takes, the less chance there is of surviving.
There has been no communication with the miners and contractors at the horizontal mine at Atarau, near Greymouth, since a methane gas explosion cut power and blew out ventilation fans at 3.45pm on Friday.
The warning came at news conference on Tuesday morning where the officer in charge of the rescue efforts revealed a Defence Force robot was sent into the mine at 6am but broke down after travelling about 500 metres.
Mr Knowles said he was told two hours later the robot short-circuited when water got into it. Requests have now been made to the United States and Western Australia for more sophisticated robots.
Rescuers have now attached a seismic device to a tube coming out of the tunnel to monitor for tapping or other signs of life. But search and rescue teams still can not enter the mine because of high levels of volatile toxic gases.
Mr Knowles says the Defence Force robot will not be retrieved while the mine continues to be unsafe.
The father of one of the trapped men told Radio New Zealand some family members became upset when they learned of the problem with the robot.
Laurie Drew, whose 21-year-old son Zen Drew is among those trapped, says families were told the rescue team was trying to find another robot, but some were angry that a back-up was not already at the site.
The remote-controlled robot supplied by the Defence Force can travel only as far as there is clear air. It is equipped with four cameras which rescuers hoped would give an indication of the conditions underground.
But a US expert says robots are not yet effective for mine rescues and using one at Pike River Coal is a long shot. Sean Dessureault, a mine automation expert from the University of Arizona, says underground conditions are cold, wet and rough on the ground, making it tough for any robotic device.
New Zealand Mines Rescue manager Trevor Watts says his team is frustrated at not being able to begin a rescue mission.
Mr Watts says the operation has been boosted by another 18 personnel from New South Wales Mine Rescue and other experts from Huntly and Waihi, bringing the total team to more than 90.
The drilling of a bore hole through 162 metres of rock into the mine shaft is in its final stages. A diamond head drill will be used for the last 10 metres to avoid the risk of sparking which could ignite gases.
Air samples will be taken once the bore hole is complete and a camera and listening devices lowered into the mine where it is thought the men are.
The bore hole will also enable a laser device to be lowered to map the inside of the mine and check for any collapse or debris.
Tests are still showing a heating source of some sort in the mine, possibly a smouldering fire.
Mr Knowles says there will be a release of gas once the surface is broken and it will be tested immediately. Another bore hole will be drilled close by to where the men are believed to be.