International mining experts last night raised concerns about safety issues at Pike River Coal which they say may have led to the huge explosion.

An Australian gas drainage engineer who visited the site last year said operating standards were "extremely poor".

His comments were backed up by a world renowned Kiwi mining safety expert who said the explosion at Pike River should never have happened.

Neither will be publicly identified but say safety problems will be investigated in the coming weeks.

"In developed countries like the United States and New Zealand we shouldn't be having these kinds of accidents," the New Zealand expert said.

He was also referring to an explosion at the Upper Big Branch mine in West Virginia which killed 29 miners on April 5.

The mine's owner, Massey Energy, has been under scrutiny since the explosion for a string of safety violations at the mine.

The Kiwi expert said if there was a power failure and the ventilation system and fans stopped working the methane would start to accumulate.

"An electrical problem could produce a source of ignition or a spark could have come from a cutting machine hitting hard rock," he said.

The Australian expert, who still has close contacts with miners at Pike River, said he had been told the mine was flooded with methane gas about three weeks ago.

Senior staff allegedly had serious concerns about safety in the mine, he said.

"The mine had an outburst risk. This is when the gas content becomes so high the coal is broken apart in a quick release of pressure," he said.

Methane gas is odourless and is released when coal is mined.

He said there had been a number of incidents where the methane content reached high levels over the last 12 months.

"Two to three weeks ago the mine fans were out and the whole mine was gassed out.

"It's a gassy mine. When the fans stopped it took 20 hours to clear the mine," he said.

He claimed miners had bored through "high flow methane holes" without any risk assessment conducted or procedure on how to manage gas flow from the hole in place.

The Australian said the company had not yet implemented a gas drainage drilling regime which could have relieved the pressure when there is a build of gas.

He said if methane builds up to explosive concentrations, it means there has been a number of failures in the safety management system.

"If management systems and procedures were effective and adhered to, this incident would not have occurred," he said.

He said staff had "severe concerns about safety".

He said a piece of equipment had a safety circuit fault but was put back into operation within hours of being tagged out of commission without necessary repair work done.

Pike Coal management couldn't be reached last night to respond to the allegations.

But earlier in the day at a press conference, Pike River Coal chief executive Peter Whittall said the stability of walls of the access tunnel were "excellent" and at no risk of collapsing.

Whittall said they weren't aware of any hazardous circumstances in the mine on Friday.

It had recently completed commissioning the first of its two main underground fans that had increased ventilation by around 30 per cent.

Whittall's speech at the company's annual meeting last week made reference to challenges.

He said the company had not met production targets because it was hampered by the ventilation shaft collapsing during construction.

The company had to spend $7 million to reinforce the shaft.

He said the company had been trying to make up for the delays by modifying the heavy coal-mining machines and contracting extra mining equipment and labour.

"The geological challenges we had been facing for some time, continued to have a negative impact on our production forecasts, affecting the rate of underground mine development and ultimately progression to full production."

EPMU national secretary Andrew Little said he was not aware of any safety concerns at the site.

He said the proper time to raise these questions would be at a formal inquiry.