"The longer it goes on, hopes fade and we have to be realistic," said police superintendent Gary Knowles, who is co-ordinating rescue efforts.
An army robot sent inside a tunnel leading to the men broke down, he said.
Supt Knowles said rescuers were still assessing air quality in the mine in the country's south island, to decide whether rescuers can attempt to go in.
"We will not go underground until the environment is safe," he told reporters on Tuesday.
Dangerous levels of methane and carbon monoxide in the Pike River mine near Greymouth have hampered rescue efforts.
Probes are taking gas samples. A listening device is due to be lowered down a borehole, which is being drilled.
Supt Knowles said drilling team had hit "very hard rock" overnight.
BBC News
