Electricity lines companies say it's lucky that a break in the Cook Strait power link on Monday night came when electricity supplies are plentiful.
New Zealand has so much electricity at the moment that Meridian Energy has been spilling water from its hydro dams in the South Island.
Transpower says the failure was not a catastrophe but the sort of technical failure that happens two or three times a year.
It says it would not normally matter except that the back-up system needs replacing.
But the Electricity Networks Association says there could have been brownouts or blackouts if the outage happened during a dry year affecting hydro dams in the South Island or when a major plant such as Huntly was out of action.
The fault will take up to seven days to fix.
The present back-up cable transmits less than a quarter of the amount of electricity as the main cable.
A new back-up system will be ready by 2012.