The head of Transpower says the failure that caused a mass power cut affecting hundreds of thousands of North Island homes and businesses won't happen again.
A botched fault simulation carried out by the national grid operator led to power cuts for one out of every 10 customers in the North Island for almost two hours on Tuesday.
The problem occurred during a routine test on a HVDC link, a $700 million line between the North and South islands known as the Cook Strait cable, due to be completed in December this year.
Transpower chief executive Patrick Strange said that as the problem occurred during a test on the link, engineers were on hand to fix it straight away.
Dr Strange told Radio New Zealand's Morning Report programme on Wednesday that Transpower knows the fix that it has to apply to stop the problem, and it would not happen again.
"There's always risk on a power system but that's why we run these very extensive tests. It's really unfortunate that it happened yesterday and we certainly apologise for that.
"But, you know, if it had to happen, much better to happen when the engineers are around (and) we can respond very quickly as we did yesterday."
Dr Strange said the electricity cut probably cost the operator $1 million.
Transpower said it was simulating a fault by deliberately dropping a piece of piano wire on high voltage lines - a test it has done before to mimic a lightning strike.
State-owned Enterprises Minister Tony Ryall says it is Transpower's responsibility to ensure the security of electricity supply.
Mr Ryall says ministers have asked Transpower for a full report on the cause of the cut and what is being done to prevent a recurrence.
