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Southern hydro lake levels are so low national grid company Transpower is only a few days away from issuing a "watch" alert to the electricity industry.
"We are right on the cusp of the first alert curve," Transpower general manager of systems Kieran Devine, of Wellington, said yesterday.
"Two storms are expected this weekend bring the chance of decent rain in the right places, but if the rain does not come we will make a decision next week about whether to raise the alert from 'normal' to 'watch'."
A watch alert did not mean there would be winter power shortages, he said. Rather it meant the electricity industry and power consumers were aware hydro generation in the South could be further reduced to conserve water.
Rainfall in the central and southern South Island has been well below average for the past five to six months, affecting the controlled and uncontrolled lakes which feed power stations at Manapouri, and on the Waitaki and Clutha Rivers.
Meridian Energy, which operates the Waitaki and Manapouri stations, and Contact Energy, which operates the two dams on the Clutha, have already cut generation to conserve levels.
While the South was still generating about 22GW of electricity daily, about 6GW of power a day was being generated in the North Island and sent south to supply southern consumers, he said.
The amount generated in the north and sent south was "gently rising" as demand rose in the South with the arrival of winter.
The last time there was a serious power shortage in New Zealand was in 1992 when street lighting was rationed and households endured hot water restrictions. Consumers were also asked to save power in 1998, 2001, 2003 and 2006, but each time rain came soon enough to head off any serious problems.
Mr Devine said he was "still mildly confident" New Zealand would get through the winter without electricity shortages.
Southern lake levels
Pukaki ... 51% of maximum operating level
Tekapo ... 49%
Hawea ... 43%
Wanaka ... 20%
Wakatipu ... 13.6%
Te Anau ... 45%
Manapouri ... 45%