KEY POINTS:
Hopes are high that heavy rain predicted in the South Island this week will bring desperately needed replenishment for the hydro lakes.
The MetService is predicting up to 350mm of rain could fall in the south as a front moves up the island in coming days and stalls, as expected.
"It's probably the best opportunity we have had for significant spill-over rain for a long, long time," said forecaster Andy Downs.
Tekapo and Pukaki, two crucial southern hydro lakes, are only 35 and 29 per cent of average capacity for this time of the year as fears of electricity shortages over winter grow.
While a ridge of high pressure over northern New Zealand is expected to keep things settled in the North Island, it is the active front moving on to the south of the South Island today that provides the best news.
The front is expected to stall over the Westland region, and heavy rain warnings are in place for Fiordland and the Westland Ranges through to Saturday.
"Ahead of that front is going to be a band of quite heavy rain, intense rain, affecting the West Coast," Mr Downs said. "And of course the good news is that whenever there is a lot of rain on those sides and on the Southern Alps, there has got to be some spillover [into the hydro lake catchments]."
The air dragged in from the Tasman Sea would be both moist and "relatively mild for the time of the year".
This meant much less of the rain would be turned to snow, and "it may even actually melt some of that recent snow they may have had".
The amount of rain that falls will depend a lot on where the front does stall, Mr Downs said.
A Meridian Energy spokeswoman said the predicted rainfall would certainly provide some welcome relief "however we don't count on it until it falls".
"We continue to act cautiously and responsibly."
