Vector, which manages the pipeline, said it was working to fix the problem, but it did not know how long it would take to restore gas.
Spokeswoman Sandy Hodge said a team of geotechnical scientists was at the site trying to establish where the leak occurred. Engineers were working out solutions.
"So what we're doing is working off several models and scenarios so as soon as we know what we're dealing with we have the solution and can fix it quickly," Ms Hodge said.
Household customers had not been asked to limit their gas use.
Late last night, the Government said it had offered Vector any assistance it might need.
Genesis Energy has switched its Huntly power station from gas to coal.
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.
Auckland hotels have also restricted their gas use, and if the pipeline is not repaired quickly, guests may have to take cold showers.
A spokesman for Accor, which owns nine hotels in Auckland, said it had enough gas to keep it going for the next few days. It would evaluate the situation when more information was available from Vector.
Auckland University has stopped all gas use in buildings other than those used for accommodation.
The gas pipe leak couldn't come at a worse time for Fonterra farmers, who are at the height of a record "spring flush", as the new milking season is known in the industry.
The Waikato and Northland district councils have urged farmers dumping milk to keep it out of the water supply and use their effluent systems to disperse it across farmland.
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.
The Waitemata and Counties Manukau health boards are to decide this morning whether elective surgery will have to be cancelled because of the limited linen supplies.
THE COST
* Farmers throwing away $20 million worth of milk a day.
* Waikato hospitals asking patients to bring own pyjamas.
* Elective surgery could be cancelled in hospitals north of Taranaki.
* Auckland hotel guests may soon have cold showers.
* Auckland University has cut gas to all buildings not used for accommodation.
* Household customers not yet affected.
