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National power grid operator Transpower has apologised to the Auckland City Council for power cuts which brought parts of the city to a halt last week.

Transpower chief executive Dr Patrick Strange, gave a meeting of the council's city development committee a debriefing on the power cuts which affected more than 70,000 homes and businesses when a transformer failed at the Auckland suburb of Penrose, just south of the city centre on February 3.

Further power cuts followed, some initiated by Transpower staff to lighten the load on an already overloaded supply.

Businesses already struggling in an economy in recession, said the cuts cost millions as the power failed several times in homes and businesses from Newmarket to Mt Wellington, including the eastern suburbs.

The failure also caused raw sewage to spill into Auckland's Waitemata Harbour and onto city beaches when pumps failed.

After the cuts Prime Minister John Key committed the Government to move fast to help fix the city's vulnerable power supply.

"We've made it quite clear we'll spend what looks like tens of millions of dollars to upgrade that equipment immediately," he said.

Committee chairman Aaron Bhatnagar said Dr Strange told the meeting he wanted to make sure the city's power supply was secure.

"Once he has got that out of the way he said he would be happy to talk to the Transpower board about how he could possibly put things right or some sort of gesture to say sorry for the environmental damage."

He said that could be repair work to some of the city's beaches damaged by sewage.

However, Mr Bhatnagar said he was not convinced by Dr Strange that the city's power supply was secure.

"I am reassured they have got a significant level of spend but I would still like to see more assurances about how they can offer more redundancy in the system."

He said that redundancy would mean if a transformer, substation or a cable failed, there were other options to pick up the load.

He was encouraged by a "subtle" Transpower move to standardise equipment, and phase out no longer used equipment, making the system far more flexible.