The Maui gas field has celebrated a double birthday.
The field was discovered off the coast of Taranaki 40 years ago and has been in production for 30 years.
Shell chairman Rob Jager says the gas field has transformed New Zealand's economy.
It was large even by world standards when discovered.
Maui has produced so much gas, it could have fired the 1000 megawatt Huntly Power Station for 150 years.
The field is now nearing depeletion, but Mr Jager says research is underway on whether its life expectancy of another five to 10 years can be extended.
The research will examine the extent of gas pockets that have not yet been produced, such as gas from lower rock layers.