GREYMOUTH, New Zealand — Rescue officials are preparing to send a high-tech robot into a mine tunnel to transmit pictures and assess toxic gas levels three days after an explosion left 29 miners missing underground.

Police superintendent Gary Knowles, the rescue controller, said Monday that the New Zealand defense department had upgraded the robot so it would not create a spark and accidentally ignite a fresh explosion in the mine.

But they were still waiting for gas conditions to stabilize further before sending the robot in, Knowles said.

The dangerous gases and underground heat have kept rescue teams from entering the mine ever since the Friday afternoon blast, and there has been no communication with the 29 missing men.