More mine rescue specialists were being rushed to New Zealand Tuesday as the grim wait to search for 29 trapped miners entered its fifth day.
The expected arrival of 18 personnel from Australia would take the number of rescuers and support staff waiting at the mine entrance for volatile gases to subside to more than 100.
There has been no contact with the missing miners since an explosion Friday and rescuers say conditions in the Pike River coal mine remain too dangerous to mount a search.
"I can't express the frustration that our guys feel at not being able to deploy underground. It is heartwrenching," New Zealand Mines Rescue boss Trevor Watts told reporters.
He said they were totally reliant on the gas readings from the mine on whether or not they could go underground to look for the men.
It was not so much the toxic nature of the gases in the mine, but whether these could ignite and create a further explosion, he said.
"The conditions that our rescue personnel are going to face are potentially going to be hostile," he added.
Watts already has 65 rescuers and 30 support personnel ready to enter the mine and tests on the gases are being taken every half hour as they search for a window of opportunity to enter.
"We have got highly trained and experienced mines rescue personnel waiting to go and collectively, between ourselves, the police, the mine management, we are unable to do so on the back of expert analysis around explosibility of gases," he said.
"All our operational plans and risk assessment processes are completed and all our teams are being briefed as they rotate through a 24-hour period on those operational plans. Everything is ready to go."
Watts said the large number of personnel was needed because of the difficulty in recovering the miners when they get the all-clear to enter the mine.
Each rescuer will be carrying about 25 kilograms (55 pounds) of breathing apparatus and other equipment on a near three-kilometre uphill walk to where the missing miners are believed to be.