The leak was found in the remote White Cliffs area, north of New Plymouth on Monday, forcing the pipeline to close.
The pipeline links the Maui gas field in Taranaki to the upper North Island and is managed by Vector, the technical and system operator of the pipe.
Hospitals, power stations, universities and other large-scale gas users were last night urged to reduce their usage.
Challenging fix
Vector chief executive Simon Mackenzie said his morning the leak was caused by a weld crack and creasing on the pipeline.
He said it was thought to be the result of ground movement.
A team including geotechnical engineers were going through the painstaking process of drilling 30cms at a time to reach the leaking pipeline, he said.
They would then repair it in an operation expected to take at least two days, he said.
Acting Energy Minister Hekia Parata said there was not yet any estimates of the economic damage being caused by the leak.
She urged residential customers to curtail their gas use by measures including taking short showers and using the oven instead of a gas stove to cook.
Mr Mackenzie said evidence of the leak was first spotted by a farmer on Saturday, but Vector workers could not find a problem.
They were called back on Monday, when the leak was identified. The company then had to isolate gas on the Maui pipeline to find out where the leak was.
"It was reasonably evident given some basically bubbling to the surface of the gas. Once the gas pipe was isolated ... we then commenced excavation.''
He said the "extremely challenging'' excavation efforts was carefully removing gas from the lines because otherwise it could cause a major explosion.
"The key issue we are focused on at the moment is to not disrupt the land as such that it gives rise to a compounding of the problem.''
He said civil aviation had been notified of the problem because the high-pressure gas gets exhausted up into the atmosphere.
$20 million of milk spilled a day
Fonterra farmers are being asked to dispose of the milk in their paddocks and avoiding the waterways.
A Fonterra spokesman said the effect of the closure on the firm and its farmers, who are being asked to throw out all their milk, had been "huge".
About 30 million litres would be lost each day the closure continued, at a cost of of $20 million a day, "so there is a big impact for us".
Fonterra is able to process only five million litres a day at its Waitoa and Te Awamutu plants, which can run on coal.
Hospitals impose BYO PJs rule, consider cutting elective surgery
Auckland's Middlemore Hospital will not be warming the non-clinical sections of its building today, says spokeswoman Lauren Young.
"Luckily it's not cold.''
She said the hospital was going "okay'' and assessing the situation on a day-by-day basis.
"We're trying to conserve our sterile supplies and our linen. We're asking staff to be careful.''
All elective surgery patients have been transferred from Manukau Surgery Centre to Middlemore, she said.
"It's just that the heating and sterilisation at the Super Clinic is reliant on gas and we can't have operating theatres that aren't warm.''
She said the hospital wasn't yet asking people to bring in their own pyjamas, but "it could come to that''.
Spotless, which cleans linen for all the hospitals operated by the Auckland, Waitemata and Counties Manukau District Health Boards, has shut all of its Auckland centres and moved its operations to Hamilton.
This means the centre at Waikato Hospital will be swamped with four times its normal workload and has started working all day and night to cope with the extra linen.
Until the gas supply is restored, only sheets, pillow cases, blankets, linen bags, sterile supplies and bassinet sheets will be washed.
Auckland DHB spokesman Mark Fenwick said its hospitals had about two days of gas supply left, after which they would have to start using diesel.
"We're lining up some contingency plans, like restricting heating in non-clinical areas," he said.
"The priority area for us to keep going is the central sterile supply area, which is a critical part of the hospital."
North Shore Hospital swapped to diesel power immediately after being notified of the closure.
