The ventilation shaft at the Pike River coal mine has been capped, just over three weeks since the first explosion there.
Helicopters were used to fit into place two 770 kilogram steel plates over the ventilation shaft and the operation was complete shortly after 2pm on Sunday.[image:731:half:right]
Police superintendent Dave Cliff says capping the top of the ventilation shaft stops oxygen from entering the mine, stabilising the conditions inside it.
He says the cap is permanent and is secured by sandbags at the moment, but will be concreted over.
Mr Cliff says the cap also improves the effectiveness of the GAG machine which is working to improve the mine's atmospheric stability.
He says the gas levels inside the mine will be carefully monitored.
Mr Cliff says if the results show there is a stable environment, the GAG engine will be shut down temporarily for maintenance.
He says with the cap in place the GAG unit may not need to run continuously and it will only be operated as oxygen levels rise.
Mr Cliff says a floxal nitrogen-generation unit has been sourced from Australia.
The machine will pump pure nitrogen into the mine, cooling it down.
He says the nitrogen will have the effect of ensuring the mine is inert so that oxygen can't enter and there's no possibility of any further explosions.
Mr Cliff expects the machine will arrive in Greymouth from Brisbane later this week.
Twenty-nine men are dead in the mine following a series of explosions since 19 November.