Lake Pukaki North of Twizel is one of the major Hydro lakes.
Records are under threat in a clash of weather systems expected to provide a desperately needed boost to dwindling hydro lakes.
A high pressure system to rival the most intense ever recorded in New Zealand is hovering over the South Island, blocking an active front that may drop
about 500mm of rain before easing tomorrow - more than twice the rainfall expected from Hurricane Gustav in parts of the United States.
Northwest gales gusting up to 140km/h were also predicted, with the potential to damage trees and powerlines. Winds averaging 100km/h were recorded off Fiordland's coast yesterday morning.
The North Island is expected to get its own heavy rain in some places on Thursday and Friday as a separate front hits.
With the southern hydro lakes so low in the south that some may be lowered below minimum operating levels, the rain was coming at a crucial time.
The south's Waitaki hydro scheme alone can generate a quarter of the country's power.
MetService severe weather forecaster Erick Brenstrum said the hydro lakes would benefit from the heavy rain spilling over "without a shadow of a doubt".
"It's just a question of how much. I think it is going to be significant."
The warmer nature of the rain, brought by winds from the north, would also help melt built-up snow.
Hydrologist Dave Stewart said the rain was "extremely timely" with North Island electricity production working "flat out" to send power south.
"A large boost will take a hell of a lot of pressure off."
Mr Brenstrum said the high pressure system over the south was expected to have a central pressure of 1049 hectopascals "which is extremely high for this part of the world".
The centre of this system was over the sea, but the highest pressure over land recorded in New Zealand was 1046hp, in the late 1800s.
Meanwhile, a climate summary released by the National Institute of Water and Atmospheric Research confirms August was, like July, an especially wet and stormy month.
In the North Island, from Auckland south with the exception of Gisborne and Hawkes Bay, rainfall totals were more than one-and-a-half times the normal amount for the month.
Temperatures across the country were close to average.
