KEY POINTS:
Power planners should know today if forecast heavy rain has helped replenish hydro lakes that are standing at less than half average levels.
A slow-moving front is forecast to dump up to 350mm of rain over 48 hours in critical hydro catchments from South Westland to the Aoraki-Mt Cook area.
In spite of the forecast, the $2.5 million television adverts promoting power saving will start on Sunday. Power companies are paying for them.
Transpower said it "must continue planning for the what-ifs until we are completely confident that we are past the risk of any shortages".
Chief executive Patrick Strange said power savings of up to 2 per cent were being achieved by householders and big industrial users.
"We expect to see this increase under this programme if all New Zealanders play their part in comfortably reducing their power use - we do not want people to turn off heaters and go cold under any circumstances."
Transpower is co-ordinating the response to threatened shortages and has resisted calls to set an overall savings target. Demand figures it will publish next week will show a regional breakdown of savings.
It was drizzling in Fiordland yesterday afternoon and MetService said it was "moderately" confident the forecast heavy rain of up to 35mm an hour would arrive.
A succession of fronts throughout summer and autumn had failed to deliver the rain they promised, leading to low lake levels and threats to power supplies given that nearly two-thirds of the country's power comes from hydro sources. Yesterday, they stood at 49 per cent of the average.
MetService severe weather forecaster Bob Lake said all computer modelling pointed to the forecast rain arriving this time. It would also be relatively warm rain that was unlikely to be locked up as snow.
Rain Effects hydrologist Dave Stewart said Lake Te Anau and Lake Manapouri, which feed the country's biggest hydro station, could start to fill by later today. The impact of the front on Lake Tekapo and Lake Pukaki, which store about 60 per cent of hydro capacity, would be apparent by Tuesday.
"We've had lots of threats of heavy rain that haven't come to anything. If this one occurs - and it does look likely at the moment - we should see a significant change in our storage situation."
