Electricity no longer used at the Tiwai Point aluminium smelter could power up to 776,000 households but it's uncertain what will happen to bills.
The smelter, which will close next year, uses up to 14 per cent of the country's generated power but transmitting that efficiently to the North Island will mean a costly upgrade of the National Grid over several years.
Consumer NZ says the increase of supply against now flat demand should ''all things being equal'' result in lower retail prices for consumers.
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"However, as always in the electricity sector, nothing is simple. There are options open to the electricity industry that could see non-renewable generation reduced or switched off, accelerating decarbonisation, but reducing capacity gains," said the organisation's chief executive Jon Duffy.