Frustrations have boiled over as efforts to find 29 trapped miners were delayed by the failure of a robot, which some argue should have been foreseen.

Families of the miners caught in the Pike River mine after Friday's explosion reacted with disbelief and anger when told at a briefing yesterday that water falling on to the robot had caused it to malfunction.

It was sent in to investigate inside the mine and to try to locate the miners.

The New Zealand Defence Force robot only got 550m into the mine when it malfunctioned - and people have questioned why it was not adapted to cope with the water.

"In this instance it was effectively hit by a waterfall, which then knocked it out," said Defence Minister Wayne Mapp.

A second Defence Force robot has now been brought in ahead of the arrival of more advanced robots from overseas.

Lawrie Drew, whose son Zen is among those trapped, told the Herald the initial reaction when families were told of the robot failure was laughter - "and then the emotion came out".

"[There was] frustration because we can't understand why they didn't have another [robot] on site. It just lessens our confidence in them. We are still just hoping and we really want the next robot to do the job."

Grey District mayor Tony Kokshoorn called the news of the robot failure a "kick in the guts".

"We were hanging our hopes on the robot because it can get around anywhere in the mine and tell you where our miners are and how they are. Without rescuers it was the next best thing. It's history. Imagine how that went down.

"People really started to question the procedures."

Carol Rose, whose son Stuart Mudge is among those missing, wasn't at the briefing about the robot's failure and, from what she saw, she was glad she didn't go.

However, she knew from her son how difficult the terrain in the mine could be.

"I guess that was why they were always going to be up against it."

News of the robot's breakdown was met with disappointment and criticism on the streets in Greymouth and across New Zealand.

Herald readers slammed the operation as a "balls up" and said it made New Zealand look like a "banana republic".

Dr Mapp said the robot was not designed to go into a mine, but could operate in rain. His spokesperson said it would be up to the people on the ground to make the call on sending the second robot into the mine.

The robot was designed for "mine disposal" and, if used, would be adapted to travel 2km into the mine.

"They'll put extra batteries on them ... they've put gas detection equipment on them and they already have two cameras."

Meanwhile, senior police detectives from Christchurch have arrived in Greymouth to begin investigations into what has happened at the mine.

With the Department of Labour, their investigations will explore whether there were failings by the mine company that could lead to a prosecution.