Key Points:

Vector has deliberately cut electricity to the Auckland suburb of St Heliers on directions from Transpower, to lessen the demand on power supply.

Other suburbs which suffered a major outage yesterday have today also suffered outages because of overload on the system.

Power was restored at about 2.40pm but Vector spokeswoman Philippa White said further cuts could not be ruled out tomorrow.

"Peak demand has now dropped below the maximum allowable load Transpower had provided Vector so all customers have had power supply restored," Ms White said.

She said before the rolling outages, Vector cut hot water to suburbs from Onehunga to Mt Wellington and Newmarket to Glen Innes and other eastern suburbs but it was not enough to reduce the load.

Ms White said hot water will not be restored until later tonight.

nzherald.co.co.nz readers have told of black-outs today in Mt Roskill, Three Kings, Meadowbank, Ellerslie and Glendowie.

Prime Minister John Key this afternoon said the Government would "immediately" invest millions of dollars in upgrading equipment.

A recorded message on Vector's fault line informs customers: "Due to the Transpower fault in Auckland on Tuesday, Vector has been instructed to implement rolling outages in your area. We apologise for any inconvenience and power should be restored in two to three hours."

Ms White said the decision on where to cut power this afternoon was based on the outage's likely impact, taking into account factors such as the area's zoning and the duration of the outage.

St Heliers is a mainly residential suburb.

"It's always a difficult choice to make," Ms White said.

She said other areas could be cut off later today if the savings were not met but whether further areas would have their power cut was up to Transpower.

Ms White said the cut to St Heliers could last two hours.

Wendy Caspersonn from the St Heliers Bay Village Association said retailers and cafes were shutting shop.

On learning that the power had been cut deliberately, she said businesses would have appreciated some warning.

She said the area had been hit with a "double whammy" after the power outage caused a sewage spill nearby.

"Obviously people come up off the beach, go to the restaurant, go to the cafes, go to the ice cream shop, see something and come back and buy it," Ms Caspersonn said.

She said it was unfair that St Heliers was targeted because there were a lot of businesses there.

"In this economic climate, everyone is feeling pretty upset," Ms Caspersonn said.

"Everyone is paying high prices for a commodity that they're not receiving," she said.

The power went out for up to two-and-a-half hours yesterday afternoon in east Auckland when a transformer broke down at the Penrose substation, affecting 74,000 homes and businesses.

Prime Minister John Key said he could not guarantee power outages would not occur but that the Government would invest in upgrading equipment.

"I don't think any politician can guarantee a piece of kit won't fall over," he said.

"But it's a legacy issue we've inherited... it is my understanding the minister [Energy Minister Gerry Brownlee] has been already in contact with Transpower today and quite clearly we will spend what looks like being tens of millions of dollars to upgrade that equipment immediately."

Conserve power

Earlier today, Transpower asked hospitals, homes and businesses in eastern Auckland to conserve power or face further power cuts.

Chief executive Patrick Strange said yesterday's outage was "unacceptable".

"We can appreciate the inconvenience this has caused. Right now, we're focused on keeping the lights on," Dr Strange said.

Dr Strange said three transformers usually took the load at Penrose but yesterday one of them was out for regular maintenance. Normally, two transformers could cope but this time one failed, causing the remaining one to trip.

He said Penrose was relying on one transformer while staff worked round the clock to try to have a second one in use some time today.

A new replacement transformer had been ordered to reinforce Penrose and a major upgrade at Otahuhu was under way, he said.

He said there was a $6 billion upgrade programme under way in Auckland at the moment but it would take time.

Impact of outage

Yesterday, power was gradually restored, with Newmarket and Sylvia Park shopping centres being resupplied after 90 minutes.

Orakei, St Heliers and Glen Innes had power supply resumed by 3.30pm.

Shops in Newmarket and Remuera closed their doors after the blackout began, four people were freed from lifts, operations were cancelled at Green Lane Hospital and Ascot Hospital and motorists proceeded cautiously through deadened traffic lights at intersections across the city from Panmure to Mt Albert.

"It was set to be a big day for Newmarket, but the rug was pulled out from under us," said a furious Cameron Brewer, who heads the Newmarket Business Association.

"When you consider everything from wasted restaurant meals, employees unable to work, wiped computer documents and missed electronic sales, the cost would have to be several million dollars.

"Given the cost of power, business now deserves a full explanation," said Mr Brewer.

Safety advice is being issued after raw sewage was discharged into the Waitemata Harbour following the power cut. Auckland Regional Public Health Service is advising people to take extra care with food and water.

The raw sewage was discharged after power to the Orakei and St Heliers pumping stations failed temporarily.

Medical officer of health Dr Cathy Pikholz advised people to keep away from any sewerage overflows, and to avoid swimming close to St Heliers Bay and Okahu Bay for the next 48 hours. Dr Pikholz also advised people not to collect shellfish from the affected areas.

Auckland City Council has put signs up at the beach, advising people not to swim at the beach for 48 hours.

City development acting general manager Ludo Campbell-Reid said the council has undertaken water samples yesterday afternoon and this morning but results are not yet available.