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An outlook of dwindling hydro lake levels means the South Island may be unable to come to the rescue as the North Island faces electricity shortages.
Wholesale power prices in the North Island surged to almost eight times that of the south on Friday, following an equipment breakdown and unsuitable conditions for power generation.
Because of a lack of reserve generation, any such hiccups were pushing the North Island power supply right to the edge, said energy expert Bryan Leyland.
"Basically we have run down our reserve generating capacity to the extent that not much has to go wrong to put us in a very dicky situation," Mr Leyland said.
Traditionally, the North Island has been able to rely on electricity generated in the South Island flowing north to cover any shortfalls, but with drought-like conditions expected over the coming months, the hydro lakes may not have the capacity.
"If we do get a dry year, and in which case the South Island needs to be propped up from the North Island, what we know is the North Island hasn't got much spare capacity to send south."
While the hydro lake levels are currently reasonable, at about 86 per cent of average storage, Niwa's outlook until March is for low flows into the lakes.
A La Nina weather pattern meant more northeast winds and relatively little rain in the west of the South Island, said hydrologist Dr Alistair McKerchar.
Mr Leyland said the Cook Strait cable, which carried electricity between the islands, could carry 700 megawatts of electricity but was often "constrained back to 400 megawatts because we don't have enough reserves to cover in case it trips off."
A piece of equipment at Contact Energy's Taranaki combined-cycle power station that failed last week was due to go out for an overhaul next week for almost two months, further raising concerns.
Other North Island generation relied on wind, while water temperature in the Waikato River could compromise generation at the Huntly power station.
Mr Leyland said there was an ongoing risk of an electricity shortage, similar to that experienced in 1992 when the country last faced a major shortage.
"You can't say it's high at this stage. But [we are] certainly aware of it. There's a high risk of a 2001-type shortage, which was quite uncomfortable.
"June-July-August is when it will bite."
