Mines Rescue teams expected to complete the installation of a temporary seal inside the Pike River mine by midnight on Thursday.
The bodies of 29 workers remain in the West Coast mine following a series of explosions that began on 19 November last year.
The third day of work to build a temporary air seal 170 metres inside the mine was delayed on Thursday morning after one gas sample indicated high oxygen levels.
However this turned out to be an equipment error and a crew was able to re-enter the mine at about 1pm.
Statutory mine manager Steve Ellis says crews were working into the night and should be able to complete the construction of the frame for the airlock.
However, pumping in concrete to create an airtight seal could take until Saturday.
Mr Ellis says conditions inside are trying. "They're working in 97% nitrogen, now they're trained to wear the breathing apparatus so it's probably not a comfortable job. It's what they're trained to do. The lads have been very keen and enthusiastic."
He says the 12 Mines Rescue men working on the recovery operation are all from the West Coast.