The Electricity Authority has confirmed Transpower is responsible for unnecessary power cuts to 34,000 consumers on one of the coldest nights of the year.
The authority's final report on the 9 August power cuts confirms Transpower's coordination and communication failures for the unnecessary outages.
At the time, power cuts hit without warning, catching thousands off guard with no time to fill their hot water bottles or find torches.
Electricity Authority chief executive James Stevenson-Wallace said the power system operator had made significant improvements since last year's event, and a monitoring programme had been put in place to ensure all the recommendations were followed.
"As contract managers for the system operator, we've learnt valuable lessons from these events and as recommended in the Ministerial Investigation, we will be holding Transpower more firmly to the rules and contracts in its performance as system operator," Stevenson-Wallace said.
In addition, an industry-wide simulation exercise would take place later next month.
"The system operator will be actioning our recommendation to lead its first industry-wide simulation exercise on May 26 - based on managing a hypothetical grid emergency where supply is forecast to fall below demand," he said.
"This is a critical preparedness exercise to put the system operator's changes to the test and to ensure its communications and processes are effective and will minimise impacts on customers in any future events."
In addition, the authority said it had made a formal complaint to the Rulings Panel regarding the alleged breaches by Transpower.
If the Rulings Panel upholds the complaint, it can impose penalties, costs or compensation and recommend amendments to the code.
An undesirable trading situation investigation related to the events of 9 August was still underway and open for consultation until 4 May.