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The South Island is bearing the brunt of a late winter threat of power shortages and a surge in power prices as lake levels in the south near historic lows.
Lake levels in the critical Waitaki hydro catchment are as little as one-third of what they normally are at this time of the year and Meridian Energy says without rain in the next few weeks an already extremely tight supply situation will get worse.
"The issue early on was getting through winter, the issue now is getting through spring," spokesman Alan Seay said.
Seay said cold and dry weather needed to be replaced by substantial rain or early snow melt.
Mighty River Power's Waikato River hydro system and Genesis Energy's Tongariro system had more water than needed but the main problem was getting enough power through a lower North Island transmission bottleneck to the South Island.
Thermal generators were based in the North Island and aside from some wind turbines, the South Island generates only from hydro dams.
Major Electricity Users Group executive director Ralph Matthes said the supply problem in the South Island would have been worse if not for Rio Tinto keeping part of its Tiwai Point aluminium smelter out of production.
Matthes said a working group of energy sector bosses has over the past week reconvened regular meetings to discuss technical issues. When the threat of winter shortages eased in mid-July the supply risk working group ended regular gatherings.
"They're looking at whatever can be done to ship electrons south," he said.
Energy consultant Bryan Leyland said South Island prices were running about three times what they normally would be at this time of the year.
The high voltage interisland link already has capacity constraints and should it fail, up to two-thirds of South Island consumers could face cuts, he said.
Transpower says it is reconfiguring the grid to relieve transmission bottlenecks.
