The November earthquake was the likely cause of a leak affecting the main water supply pipe to Wellington City.

Wellington Water staff and contractors worked all night Friday to find and repair the leak, and are back again today in the bottom of a 4m-deep hole next to the Wellington Railway Station.

"We came pretty close yesterday to sending out a call asking Wellingtonians to cut back water use to only the bare essentials," said acting Wellington Water CEO Mark Kinvig.

"There's really only one main supply pipe into central and eastern Wellington, and this was it."

Gary O'Meara, Wellington Water's general manager of Network Operations and Customer Service said it was likely the November earthquake was behind the leak.

"We know that the foreshore area suffered the worst, and our pumping stations and pipelines in the port area suffered damage in the November 14 quake. When our guys cut through the concrete road base where the leak was, they found a big hole. It's clearly been leaking for a while."

The leak, from a damaged valve leading off the main pipe, appears to have been eroding the sandy soil beneath the road surface.

It was touch and go on Friday night whether Wellington would have enough water on Saturday morning, as the two main storage reservoirs supplying the central and eastern suburbs slowly emptied out in the evening peak. Operators worked hard to reroute supply, but it was not going to be enough, O'Meara said.

"If we hadn't fixed it by 6am yesterday, we would have had to ask people to ease off. And if the leak had happened further up the pipe, near Ngauranga, we would have had major issues."

The earthquake and after effects such as this were a wake-up call for people and businesses to plan for the loss of water supply, said Kinvig.

The vulnerability of Wellington City's water supply is well known. Plans are in place to make it more resilient, including building a 35 million litre reservoir in Prince of Wales Park, and for an alternative pipeline into the city. Both these projects will cost in the tens of millions. They'll also take years to complete.

People should store enough water at home for at least seven days, to take care of their health and hygiene in case the water goes out, Kinvig said.

"In the meantime, we rely on the experience and dedication of our team. They did a great job on Friday night and early Saturday. But there's still more to do to complete this repair, so they'll be down there today completing the repair".

About 50 million litres a day goes through that part of the network.

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