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One of the country's biggest generators is worried about the risk of power shortages next winter.
Genesis Energy chief executive Murray Jackson said there appeared to be very little room for mishaps given the projected demand.
Peak supply to the North Island could be down as much as 8 per cent next year if a part of the interisland power link stays out of action and a power station at New Plymouth remains closed.
National grid operator Transpower yesterday said supply should be "tight but manageable".
Switchgear known as "pole one" was out of action due to concerns about its viability and safety and Transpower's system operator Kieran Devine said this meant margins next winter would be tighter.
Further details are due tomorrow when an electricity sector working party publishes projections for winter.
Low water levels in South Island lakes have caused spot prices to soar during the past two weeks but hydro levels in early summer traditionally have no bearing on the following winter because of inflows from rain.
Genesis is the country's biggest thermal generator from its Huntly power station and Jackson has expressed concern about the push to sustainability endangering security of supply. He said that coal and gascould underpin hydro generation throughout January and early February until industry started up.
"Will it rain over summer? Your guess is as good as mine. I don't expect that drought to continue through to next winter - God help us if it did."
He said the test would come if large generating machinery was knocked out by an accident in June or July.
"Is it manageable? Of course it is if you don't lose any of your large generating plants."
The supply problem had been aggravated by the shutdown of Contact Energy's ageing New Plymouth gas station by an asbestos scare with no recommissioning date set.
The Electricity Commission - charged with providing the framework to guarantee reliable power supply - decided that there was no need to procure any reserves for this year or 2008 in addition to the diesel-fired 155-megawatt Whirinaki power plant that can be fired up in an emergency.
A spokesman said there were systems designed to cope with "generation and transmission issues".
"We've been able to manage supply in the last few years without any significant issues," he said. In the meantime the threat to supply was low risk in spite of the falling lake levels that have resulted in wholesale power prices increases of 30 per cent to $63 MW/h during the past week.
Devine said it was not until February or March that hydro lake levels became critical.
"The risk of running out [now] is zero - it has no impact on the winter because it is too far away."
A spokesman for the country's biggest hydro generator, Meridian, said the low inflows were not causing concern.
"If we were at this stage in March going into winter we would be concerned."
Inflows into the main storage lakes, Tekapo and Pukaki, had tended to be later in summer in the past few years.
