There has been a third explosion at the Pike River mine this afternoon.

Pike River Coal chairman John Dow said there was nobody at the mine portal during the explosion and no injuries.

"We've consistently said that this is a potentially explosive environment, this confirms that," said Mr Dow.

"The plans we have in place will continue because we've expected this."

Recovery teams would not have known about the explosion, he says.

"The only reason we know it happened was because we can see it on the CCTV.

"The only way we know that it happens is that it there is smoke in the portal."

The blast happened at 3.39pm, five minutes short of one week since the initial blast. Today's blast lasted 23 seconds.

A group of workers had gathered at the mine to observe that request, when the explosion occurred. It was not known whether the commemoration was disrupted.

Mr Dow said it was possible the workers had not been aware of the explosion.

Families of the dead men due to be bused to the mine tomorrow would still go, reporters were told.

They would not be taken to the mine portal or into an unsafe zone, Mr Dow said.

The explosion would not affect attempts to recover the bodies of the 29 mine from the depths of the coal mine, he said.

Video footage shows extensive damage from second blast

Earlier today video footage taken from a robot confirmed the second blast at Pike River was more intense than the first, which trapped 29 men in the West Coast coal mine a week ago.

The robot got 1575m into the mine tunnel, where it found debris much more severe than when Daniel Rockhouse and Russell Smith escaped from the initial blast last Friday.

A coal conveyer belt battered last week "was very much more damaged to the point of not being able to be passed," Pike River chief executive Peter Whittall told a press conference.

"There's a lot more destruction in there now than there was after the first blast",

Video footage indicated that Wednesday's secondary explosion had been was a "much more violent blast", he said.

A resolve to remove the bodies of the 29 men remained intact, despite gas levels fluctuating between low and explosive, he said.

Fibre optics and video cameras were being moved in to help monitor the site, though a second drill hole would not be started until it was safe, given "a very gassy environment".

A jet engine that could blow inert gas into the mine had arrived from Australia this morning and was being assembled at Hokitika airport, from where it would be trucked to the mine.

"No decision as to whether to use it or not has been made at this stage," Mr Whittall said.