With hydro lake levels falling, spot power prices rising and hydrologists predicting little short-term change in inflows to southern reservoirs, there are fears of a return to the big blackouts of the dry autumn of 1992.
About 400 experts at last week's national power conference were divided over the issue, Genesis public affairs manager Richard Gordon said.
"Fifty per cent of those in the room raised their hands (to say we were heading for blackouts this winter), and when asked who thought the opposite, the other 50 per cent raised their hands," he said.
Storage levels in hydro lakes -- which are used by hydro power stations to produce around 60 per cent of New Zealand's electricity -- are at the same level as they were when the country last faced a major power shortage in 1992.
During that shortage, Southpower went so far as to pay large consumers around Christchurch to use LPG or diesel instead of electricity.
The storage levels are now around 70 per cent of average for this time of the year, and the National Institute of Water and Atmospheric Research (Niwa) is not forecasting significant rainfall over the next three months.
Trustpower, Contact, and Genesis are now running more expensive coal and gas-fired stations at full-throttle to compensate for the water shortages.
Electricity Commission chairman Roy Hemmingway said the situation had become a matter of concern.
The commission was established by the Government in 2003 to oversee the electricity industry.
"The longer we go with lake levels staying low the risk increases, but at this stage we are very likely to get through the winter without shortages, he said.
"Hydro lakes are relatively low, but they're certainly not at record lows."
A Niwa hydrologist -- who didn't wish to be named -- said they were predicting "normal or below stream flows in the western South Island, which are basically the inflows to the hydro lakes".
He added the situation would only improve after "several days of northwesterly conditions that bring a lot of rain into the West Coast".
Niwa, however, is predicting normal, or perhaps "below normal", rainfall.
He agreed if the weather conforms with predictions, reservoir levels are more likely to continue to fall.
Among those raising their hands in expectation of a winter blackout at last week's national power conference was Genesis' chief executive Murray Jackson.
Mr Gordon said Mr Jackson "thought there would be serious concerns".
When the crisis level is reches, the 155 megawatt (MW) oil-fired, dry-year reserve government-built power station at Whirinaki in Hawke's Bay kicks into action.
Aluminium producer Comalco, which uses 15 per cent of New Zealand's power, has cut production by more than 10 per cent at its Tiwai Point smelter over the next few critical weeks.
Mr Hemmingway said measures are in place to encourage the industry to conserve water and maximise thermal generation.
"However, the public should be aware that the situation may worsen and we may well be calling on them for conservation actions to be taken."