Police say work to seal surface cracks around the slimline shaft at the Pike River Coal mine is complete but it will be hours before they know if it has worked.
Workers start pumping expanding foam sealant into the cracks around the shaft about 8am today and completed the work by 1.30pm.
"It will be some hours before testing of the effectiveness of the sealing can be achieved," said Inspector Gary Knowles.
Police have described the sealing work as a "critical step" in the process towards stabilising the mine.
Previous efforts since the weekend to try and seal the cracks, to allow the GAG and Floxal machines - which are pumping water and nitrogen into the mine - to operate more efficiently, have repeatedly failed, with workers forced from the site because of rising gas levels.
Temperatures and methane gas levels inside the West Coast mine have continued to fluctuate, hindering attempts to get in and recover the bodies of the 29 miners and contractors killed in a series of explosions since November 19.