Large-scale outages included, but were not limited to, Flaxmere, Raureka and Taradale.
On Monday, a spokesperson for Transpower, which manages the national grid, said demand for electricity across the country was at an all-time high.
Insufficient generation meant it was unable to meet demand and manage a secure system, which resulted in Transpower asking the distribution companies to reduce load.
"Different companies will do this in different ways, some manage via load control on hot water, some manage via customer disconnections."
The situation was resolved by 8pm when the peak demand for electricity had passed.
In response to why the power was out in certain areas and not others, Unison relationship manager Danny Gough said in a rolling outage, areas with important loads like hospitals, healthcare centres, emergency operations centres, and lifeline infrastructure were not targeted.
"We prioritise areas, and the idea is we try to share the pain as much as possible. We don't target areas," Gough said.
"Last night we had to look at loads, and go with areas with the least number of feeders and most amount of load.
"We were required by Transpower to turn off sufficient load to help alleviate the grid emergency and our inability to turn off solely one customer type (e.g. only commercial or only residential) was compounded by the after hours nature of the event making these commercial sites less feasible to achieve the required load shed.
"If the event had carried on, we would have gone on to other sites like Havelock North. It's not a pleasant situation for anyone."
Gough said the rolling outage plan outlined how load would be cycled throughout the contingent event so that no single area was unfairly burdened with outages.
"But in last night's case the event didn't last long enough (1.45 hours) to trigger the next cycle of outages, allowing the first areas to be restored," he said.
Gough said Unison could not guarantee uninterrupted supply, but would notify consumers about the next rolling power outage.
"Unfortunately we can never guarantee an 24/7 uninterrupted supply of electricity, and due to the extreme cold weather currently hitting the country we are continuing to see record levels of power demand across the country," he said.
"This record demand combined with lower than normal generation levels will continue to place pressure on the national grid, so it is important that customers conserve power as much as possible at this time.
"That said we are not expecting a repeat of Monday's rolling power outages, but advise customers to have their back-up plans in place and always be prepared for unexpected or short notice power outages."
He said Monday night's outage was something to learn from.
"We will notify people before the outage, we have to get better," Gough said.
"We acknowledge the impact of last night's rolling outages on our customers, especially given the very cold conditions and timing of the outages.
"We certainly apologise for the disruption and stress these outages caused. We are working closely with Transpower to ensure we can notify our customers as soon as possible if it looks like we need to conduct any rolling outages in the future."
SIDEBAR: Five ways to conserve power-
1- Shorter shower
2- Use LED light bulbs
3- Turn appliances off at the wall
4- Make sure your home is well-insulated, use hot water cylinder wraps
5- Use timers on heaters to avoid excessive heating
