New Zealand police on Monday released the names of the 29 miners trapped in the Pike River coal mine on South Island's West Coast since Friday.

Rescue operation chief, Tasman District police area commander Gary Knowles told a press conference on Monday that the names of the miners would also be posted on the police website the same day.

Knowles said after three days without contact, the situation remained "grave" but he said drilling at the site was continuing and progress was going well.

Knowles said the rescue team remained on standby and were looking at what sections of the mine they could enter first.

A robot could be used to go inside the tunnel when the opportunity arose, he said.

Testing of the gas levels in the mine was being carried out every half hour.

"We remain optimistic," Knowles said, adding "but at the end of the day this is a search and recovery operation. We are going to go in and do our best to bring these guys to out."

Pike River CEO Peter Whittall said the team drilling over night had done a "phenomenal job" and had reached 100 meters of the 162 meters they needed to drill to.

New Zealand Prime Minister John Key, who arrived at Greymouth on Monday morning, told the media conference that these were "very testing and difficult days for Greymouth, which is a tight-knit community".

"There is no question that the families are showing high levlees of fear, anxiety and frustration. But we owe it to the men that are underground that we undertake a rescue that does not endanger their lives any further," Key said.

"We are going to get through this and do everything we possibly can to get the men out alive. That is our single focus at the moment," he said.

Testing on Sunday showed the gas levels were generally declining but fluctuating, and there is still a risk of another explosion.

Family members of the trapped miners were given another opportunity to head up to the mine site on Monday.