Drilling on a new borehole into the Pike River coal mine, where 29 miners are trapped, is in its final stages, with 20m of the 162m tunnel to go.

But it is expected to take until morning before cameras and listening devices can be lowered into the mine.

Drilling will be slowed during the last 10m, and a diamond drill bit used, to minimise the risk of sparks and another explosion.

Pike chairman John Dow said earlier today that the switch from open-hole drilling to a diamond-tipped drill was to minimise the chances of the drill igniting sparks.

"The drilling that is going on at the moment is a rotary, it's a grinding progress and could cause sparks at the bottom of the hole.

"The diamond drilling cuts through rock without causing sparks at the bottom of the hole. It's just a common sense safety procedure," Dow said.

"It's possible that the air coming up the tunnel is bypassing and going up the shaft and diluting the readings we are getting up the shaft at the moment. We need to go away from that area to where the men are."

Work will continue around the clock to complete the last few metres of drilling.

Pike River Coal chief executive Peter Whittall says the borehole will be used to take new gas samples, pictures and audio from inside the mine.

"But most importantly we'll be able to take new gas samples and have them analysed straight away."

Temperatures and gas levels in the mine are still too high for any rescue of the trapped men.

And police Superintendent Gary Knowles admitted, though police are "keeping an open mind", they're preparing for "loss of life".

"We still remain optimistic, but we are planning for all outcomes and ...also in part of this process we are planning for the possible loss of life as a result of what is happening underground," Superintendent Knowles told a press briefing tonight.

Reports from experts indicated there was still heating in the mine that may ignite.

Samples of the gas in the mine are being taken every half hour.

Mr Knowles said officials needed to establish "beyond reasonable doubt" that conditions were safe to enter.

He said preparations were also being made to insert a robot into the mine.

Tonight's message was similar to that delivered earlier in the day, when Mr Dow said gas samples from the mine's main ventilation shaft were still showing high levels of carbon dioxide and methane.

Asked if it was likely a rescue attempt would begin today, Mr Dow said: "I don't think that will happen".

Levels were not getting any worse, but they weren't getting any better either, he said.

"Once they see an improvement in the atmosphere and once they are comfortable with how that is working out and that the atmosphere is not explosive they will go in, but we are not at that point yet."

Officials weren't expecting to see pictures from the camera showing the miners clinging to life.

"Ordinarily it's in an area where we wouldn't have people working, but on this particular day there were people doing maintenance work in that area."

"It would be nice to see people huddled around an air supply waving, but honestly, the main purpose of the hole is to get gas samples."

But Mr Dow was still holding out hope for the 29 trapped miners.

"We're in search and rescue mode. We're going to get our guys out."

The trapped miners:
Conrad Adams, 43 (Greymouth), Malcolm Campbell, 25 (Greymouth - Scottish), Glen Cruse, 35 (Cobden), Allan Dixon, 59 (Runanga), Zen Drew, 21 (Greymouth), Christopher Duggan, 31 (Greymouth), Joseph Dunbar, 17 (Greymouth), John Hale, 45 (Ruatapu), Daniel Herk, 36 (Runanga), David Hoggart, 33 (Foxton), Richard Holling, 41 (Blackball), Andrew Hurren, 32 (Greymouth), Jacobus 'Koos' Jonker, 47 (Coben - South African), William Joynson, 49 (Dunollie - Australian), Riki Keane, 28 (Greymouth), Terry Kitchin, 41 (Runanga), Samuel Mackie, 26 (Greymouth), Francis Marden, 42 (Runanga), Michael Monk, 23 (Greymouth), Stuart Mudge, 31 (Runanga), Kane Nieper, 33 (Greymouth), Peter O'Neill, 55 (Runanga), Milton Osborne, 54 (Ngahere), Brendon Palmer, 27 (Cobden), Benjamin Rockhouse, 21 (Greymouth), Peter Rodger, 40 (Greymouth - British), Blair Sims, 28 (Greymouth), Joshua Ufer 25 (Australia), Keith Valli, 62 (Winton).