A petroleum conference in Auckland has been told the Maui gas field still has plenty of life.
The field off the coast of Taranaki was written off as depleted several years ago.
But the company leading gas extraction from the site, Shell New Zealand, says Maui will last between three and 20 more years, with the most likely scenario toward the upper end of that range.
Maui was discovered in 1969 and started producing for the Crown 10 years later.
Shell feared the field might run out in 2003, then in 2007. But better technology found ways of extracting low pressure gas and finding hidden pockets of gas.
Three years later it is still going strong, and Shell New Zealand chairman Rob Jager believes Maui has plenty more years left.
Mr Jager says the remaining gas is trapped between layers of water and is being accessed by horizontal drilling.
US company hopes to begin drilling soon
Houston-based oil multi-national Anadarko says it hopes to be drilling off the South Canterbury coastline next year.
Company executive Brett Dixon told the petroleum conference on Wednesday that analysis of seismic data in the area is promising, but it will not be know how much gas and oil is available until a well is drilled.
Earlier this year, Anadarko's joint venture partner Origin Energy said there could be energy equivalent to 500 million barrels of oil in two sites off South Canterbury - Carrack and Caravel - which would dwarf the Maui gas field.
Mr Dixon concedes getting access to a spare oil rig will be difficult.
Anadarko recently gained a five-year extension to its programme from Crown Minerals but is expected to start drilling in 2011.