Government officials say the Maui field should be able to keep pumping gas past 2030.
Their comments came as Shell Todd Oil Services sought approval from the Environmental Protection Authority to keep running Maui for another 35 years.
They follow several assertions in the past that Maui would soon run dry.
However, in applying for the right to keep on pumping gas, Shell Todd Oil Services revealed Maui still had 18 percent of New Zealand's gas reserves and 20 percent of its production.
Later, officials of the Ministry of Business Innovation and Employment produced more details of Maui's capacity.
They told the inquiry Maui still contained 466 petajoules of gas, a 50 percent increase on estimates a year earlier, and almost two and a half times New Zealand's total annual usage.
Pumped out judiciously, it would last for at least another 15 years.
The ministry officials did not put a monetary value on the gas but the wholesale price for natural gas is around $5.50 a gigajoule.
That mean Maui's estimated reserves of 466 petajoules would be worth $2.5 billion, minus the cost of extracting the gas and getting it to market.
Maui was discovered in the 1960s and has been producing gas since 1979 but it needs a marine consent to keep on operating under a new law governing New Zealand's Exclusive Economic Zone.