The bid to rescue 29 New Zealand coal miners trapped underground by a massive gas explosion has run into more problems as a mechanical robot broke down inside a tunnel and hard rock layers slowed progress on drilling to test the air.

Police superintendent Gary Knowles said the army robot sent in to transmit pictures and assess toxic gas levels was damaged by water and out of commission.

Authorities were urgently seeking other such robots from West Australia and the United States to replace the broken one, Knowles said.

"Safety is paramount in relation to sending people in," he said. "The levels of toxicity and air underground are still too unstable to send rescue teams in," he added.

The police superintendent's statements came as rescuers waited impatiently for a chance to test if air quality underground was safe enough for them to go in to pull out the miners, who have been trapped for nearly five days.

A diamond-tipped drill was put to work as workers hit layers of hard rock and came within 33 feet of the tunnel where they believe some of the miners are trapped, Pike River Coal Chief Executive Peter Whittall said.

The 500-foot-long shaft they are creating will allow them to sample gas levels - including explosive methane and carbon dioxide - and determine if rescuers can finally move in days after the blast.

Family members have expressed frustration with the pace of the response as officials acknowledge it may be too late to save the miners, who have not been heard from since a massive explosion ripped through the Pike River Mine on the country's South Island on Friday.

Two workers stumbled out of the mine within hours of Friday's explosion, but there has been no contact at all with the remaining 29. A phone line deep inside the mine has rung unanswered.