Here we go again. Another power emergency in Auckland, a city that's building a reputation for them.

Sixteen years ago, the lights went out in the central city for up to five weeks because of overheating cables. Then in 2006 and 2009, nearly a million Aucklanders were without power when two corroded shackles worth a few dollars each failed and a forklift driver lifted a shipping container into high-voltage lines.

There have also been the regular, more-isolated cuts due to storms.

Power outage: Penrose substation blaze

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A blaze at the Penrose substation has caused power outages which are affecting large swathes of the Auckland region.

But the latest one, cause unknown and blame unattached at this stage, is troubling because the vulnerability of parts of the system is no secret. Back in 2011, the risk of fire at substations such as Penrose was identified as possibly leading to a "high-impact failure".

Part of that substation, where yesterday's fire broke out, is having $20 million spent on upgrading it. If that part of the system had been identified as a weak point, why has the work not been done sooner?

Taxpayer and consumers' money has funded the nearly $2 billion spent on fortifying national grid operator Transpower's network through Auckland over the past seven years, and lines company Vector spends hundreds of millions of dollars more on its system. Take a look at the fixed component - Transpower and Vector's share - of ever increasing power bills and you get the picture.

The highly paid heads of these monopoly businesses are making the right noises about the priority being to get power back on before starting any blame game.

But once the electricity's back on, the public need to know where responsibility lies.

Transpower and Vector say (appropriately) they can't guarantee the power will stay on all the time. They'd have to duplicate every asset to get close to that and there's no appetite for a such a multi-billion-dollar spend-up. They say it's getting power back on quickly after a big cut that really counts.

In Auckland, they're getting plenty of practice at that.

Grant Bradley is Business Herald energy reporter.