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Transpower has been issuing daily updates of lake levels in recent times and power storage levels are well down on average for this time of year.
Lake levels are usually low at the end of summer but in recent weeks they have been low enough for generators to be concerned if autumn rains do not arrive.
National storage in April is 3488 gigawatt hours on average, yesterday it was 2624GWh, 480GWh away from the first of several risk curves Transpower monitors.
"The lakes are quite low but there have been lower years," a Transpower spokesman said.
"We monitor several risk curves so that if there was a problem we would be ready."
Storage levels have been concerning enough for the Huntly power station, near Hamilton, to have been operating at near full capacity in recent days.
Among the lakes Transpower monitors, Lake Pukaki was at almost half its historical average storage capacity, as was Lake Manapouri.
Te Anau was in a similar position, but Lake Hawea was at 219GwH, about 75% of average.
"We are not about to take any action, but we are getting near to the point where we would have to start thinking about taking some action, but we don’t know yet if that will be necessary," the spokesman said.
Most years it would rain, but Transpower operated on current figures rather than its expectations, he said.
"Where we are sitting now is the starting point for preparedness ... but where we sit now, going by the 90 years of data we have available, there usually isn’t a problem.
"But our job is to be ready, in case this is a year where there is a problem."
National storage: 64.6% of average
North Island: 56.4% of average
South Island: 65.2% of average