Transpower says it won't be compensating customers in Waikato after they were without power for more than 13 hours.
An explosion at a substation in the town of Cambridge knocked out power for 12,000 homes and businesses including farms in the area from midnight on Thursday until about 2pm on Friday.
The national grid operator says the explosion caused hundreds of thousands of dollars of damage to the station - but it's still investigating what went wrong.
Chief executive Patrick Strange told Radio New Zealand's Checkpoint programme on Friday that compensation is not being considered.
"Unfortunately we can't build a power system that's 100 percent secure and, while I hate saying it, once in a while these sorts of things do happen."
Dr Strange says staff are carrying out forensic work on the equipment to find out exactly what caused the explosion and said the switchboard had been scheduled to be replaced later this year.
Fonterra's processing plant at Hautapu in Waikato was among the businesses affected. Plant manager plant Dave Young said milk was being sent to the co-operative's neighbouring factories on Friday.
Federated Farmers said many farms in the area did not have a back-up power supply.
The power cut caused problems for retailers and several factories shut down for the day.
Call to conserve water
People are being urged to continue conserving water on Friday as the the North Island region's water reservoir levels are still critical and need to refill.
Waipa District mayor Alan Livingstone said it was a huge relief when the power went back on, but urged people to save water as it will need to be treated before use.
"We weren't able to replenish the reservoirs until the power came on. We had eight hours of water available in store but, quite clearly, we need to replenish that.
"Ideally, it would be that the residents conserve water for the rest of the day so that we can replenish those reservoirs overnight."
A tanker of drinking water was sent to Ohaupo School which had no water earlier on Friday.