Hydrologists are watching a cold front approaching New Zealand, which is set to bring significant rain to the southern hydro catchments.
The MetService expects the front to roll onto the West Coast of the South Island on Friday.
A heavy rain warning is in place for the ranges of Northern Fiordland and Westland, with spillover into the Otago and Canterbury hydro catchments.
As much as 250 millimetres of rain is predicted from early on Friday morning until Saturday. MetService is warning that rivers will rise rapidly.
The rain could be a boon for catchments around the southern hydro lakes, already topped up last week.
The lakes are now sitting about 53% of average for this time of year and any rain will help with the national electricity savings campaign.