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

Police District Communications manager Barbara Dunn said the blast occurred at 3.39pm and lasted for 23 seconds.

"It was smaller than the previous explosion and it's not going to interrupt the ability to get into the mine," Ms Dunn said.

She said it's "business as usual".

Grey District Mayor Tony Kokshoorn was not aware of the explosion.

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.

Blowing gas into the mine would make the atmosphere inert so any heat source would not trigger an explosion.

Teams of technical experts were evaluating the underground conditions with a view to "inert the mine, and make it safe to enter that way", he said.

Another group was looking how best to enter the mine once it was inert "to get our guys out and bring them back to their families".

Police Inspector Mark Harrison was in charge of the retrieval operation.

Tomorrow, families of the dead men - up to 500 people - would again be bused up to the mine, where they would be shown photographs related to the mine and the work done there.

"We're getting some photos of the mine site, we're getting some photos of the men's tags still on the board, and we're getting some other photos that we're putting up for them to have a look around the place."

Miners leave tags when they go underground, so that how many are working is known by those outside. The tags have hung there for a week, since the mine was rocked by a blast at 3.44pm.

Mr Whittall and rescue operations boss Superintendent Gary Knowles this afternoon were to join workers and rescuers at the mine, for a moment's silence at 3.44pm, marking the one week anniversary of the tragedy.

"We will have a moment's silence for our friends who are in there and for the work that's still to come to get those guys out," he said, calling on others to join him as a mark of respect for those who died.

"I would ask you just to check your watches around that time and it would be wonderful if everyone that's here and everyone that's watching this broadcast can also remember a moment's silence at 3.44 this afternoon."

Maori have performed a karakia at the mine site, alongside the White Knight Stream.

Mr Whittall said White Knight Stream was sacred to iwi, as well as a symbol of mine.

Pike River gave photos of the stream to people as when they quit the company "so to hold that karakia on the banks of the White Knight Stream was very powerful for those of us involved".