Anguished relatives voiced frustration as poisonous gases prevented rescuers from entering a New Zealand coal mine yesterday, a day after a powerful blast left 29 workers missing underground.

"If I had my way I'd be down there, I'd go into the mine myself," said Laurie Drew, whose 21-year-old son, Zen, is one of the missing men.

Rescue organisers said the level of methane and carbon monoxide was still too high to send a crew into the Pike River mine. Drew said he was frustrated by the lack of action from rescuers, who he said were giving excuses instead of finding solutions.

Two miners reached the surface after Friday's gas explosion, but there has been no word from 29 others. Police said the miners, aged 17 to 62, are believed to be about 1.2 miles down the main tunnel.

"Unfortunately it's just not as simple as putting on a mask and gown and rushing in there," the police search controller, superintendent Gary Knowles, told TV One. "It does pose a danger to those guys underground and a danger to the staff going in."

After a day of monitoring, air quality tests showed that gas levels had not dropped sufficiently and Knowles said the rescuers would remain on standby until the gases were checked again. He remained confident that the 16 mine employees and 13 contract miners had survived.

"This is a search-and-rescue operation, and we are going to bring these guys home," Knowles said.

The blast was probably caused by coal gas igniting, Pike River mine chief executive Peter Whittall said.

Electricity in the mine went out shortly before the explosion and that failure may have caused ventilation problems and contributed to a buildup of gas. The power cut continued to frustrate efforts yesterday to pump in fresh air and make it safe for rescuers, though Whittall said air was flowing freely through a compressed air line damaged in the explosion.

"We have kept those compressors going and we are pumping fresh air into the mine somewhere. It is quite conceivable there is a large number of men sitting around the end of that open pipe waiting and wondering why we are taking our time getting to them," Whittall said.

A working phone line to the bottom of the mine, however, had rung unanswered.

The two dazed and slightly injured miners stumbled to the surface hours after the blast shot up the mine's 354ft long ventilation shaft. The men were taken to a hospital for treatment for minor injuries and were being interviewed to determine what happened.

Video from the scene showed blackened trees and light smoke billowing from the top of the rugged mountain where the mine is located, near Atarau on South Island. It is New Zealand's largest underground coal mine.

The coal seam at the mine is reached through a 1.4-mile horizontal tunnel into the mountain. Whittall said the horizontal tunnel would make any rescue easier than a steep-angled shaft.

"We're not a deep-shafted mine so men and rescue teams can get in and out quite effectively, and they'll be able to explore the mine quite quickly," he said.

Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for "several days".