Key Points:

A second transformer is now working at the troubled Penrose substation, which "minimises" the likelihood of further power cuts today.

Transpower chief executive Patrick Strange said the transformer that had failed at Penrose went back live last night and had been running satisfactorily all night.

"So providing we don't have problems as the load builds on that this morning, we should not have rolling cuts today," he told Radio New Zealand.

Dr Strange said the situation was significantly improved, but consumers should continue to conserve energy.

He cautioned that bringing a transformer back into service - "and we have had to mix and match to do that there" - always carried a heightened risk that it could trip out.

"If that happened, we would be back to the risk of having to cut back demand if there was not enough saving."

Transpower spokeswoman Adele Fitzpatrick said the chance that the second transformer would now fail was low.

Aucklanders have had two days of disruption to their electricity that has left business owners and residents furious and demanding answers.

The problem is the creaky state of the electricity grid - parts of which are 70 years old - which has suffered from years of underinvestment.

But Prime Minister John Key yesterday committed the Government to move fast to help fix the city's vulnerable power supply infrastructure.

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

An upgrade cannot come quickly enough for businesses and homes in affected areas.

Network company Vector cut power in St Heliers and parts of surrounding eastern suburbs about 12 noon yesterday. The deliberate cut was made at the request of Transpower, which hoped to reduce pressure on its Penrose substation. The substation's capacity has been heavily reduced after a transformer failed on Tuesday, leaving only a single transformer to carry the load.

And when that started to peak close to lunchtime, the result was a loss of power to about 7500 homes and businesses.

Vector warned yesterday that more "rolling outages" could be needed today if Transpower's transformer in Penrose wasn't back in service.

Transpower chief executive Patrick Strange last night apologised to those who had been affected.

"The first thing to say is that whatever the faults with the system, our job is to keep the lights on and we failed to do that. We apologise to all the customers. We know the impact and we're doing everything we can to keep them on and to limit any further outages."

Last year, Transpower embarked on a 20-year transformer replacement programme and ordered spare transformers for emergencies, but they will not be delivered until towards the end of next year at the earliest.

The power cuts have irritated Auckland Mayor John Banks, who called for Dr Strange to donate a week's salary to the Auckland City Mission as a peace offering to the city.

The highest-salaried Transpower staff member - likely to be Dr Strange - last year was paid $880,000.

"I'm an Aucklander. All my focus is getting this back on ... What I do with my private life should remain private," Dr Strange said.

He said he knew people were frustrated but all Transpower could do was be totally honest about what it was doing and to make sure it didn't happen again.

Transpower setbacks weren't just restricted to the Penrose substation.

A cable that fed one of the two transformers at the Otahuhu substation - the site of the infamous 2006 power failure - also failed yesterday. Dr Strange said that while it would draw resources because it would need to be fixed, it wasn't expected to cause problems.

"That's business as usual, it's well within its load ... It happens from time to time but we're working hard to get it fixed," he said.

However, businesses and schools in St Heliers were annoyed by the cuts. Lawyer Tony Ivanson said his firm had suffered.

"I am trying to run a professional practice from the St Heliers village. Today the power was cut at 11.45 and restored 20 minutes later. Vector fails to realise the disruption they cause when they flick the switch and give no information when service will be restored."

It was back to basics at St Heliers School, where a teacher had to walk around class with a hand-held bell.

1998: Mercury Energy's underground cables feeding Auckland's central business district fail. Five weeks of blackouts and disruption follow.

2006: Cable failure at Transpower's Otahuhu substation. Much of Auckland without power for five hours.

2007: Half of Transpower's interisland link closed for safety reasons. Patch-up job done but constraints remain on shifting power between the islands.

This week: Ageing Transpower transformer fails during routine maintenance at Penrose substation. Much of city without power on Tuesday afternoon.

- With NZPA