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A software glitch left several Genesis Energy's pre-pay customers in the dark and cold during the weekend.
The problem meant customers could not buy power to heat their homes.
TV3 News reported there are 10,000 prepay clients, and Genesis would not say how many were affected.
As a cold snap hit the country, prepay electricity customers were told they could not top up, and would have to make do without power.
Genesis said it had a software problem, but it managed to get the system running yesterday afternoon.
Some customers said the system should default to power being on, not off, if the company's systems failed - because in winter customers were too vulnerable.
There are 40 outlets in New Zealand where customers can use the prepay system.