The Electricity Authority says shortcomings in Transpower's systems and communication contributed to power blackouts last month.
More than 34,000 customers lost power on 9 August when cuts were ordered because there was not enough power to meet record demand.
Electricity Authority chief executive James Stevenson-Wallace said the national grid operator took immediate action in difficult circumstances to avert widespread blackouts on a very cold night.
"However, the tool used for demand allocation and communication processes were unsatisfactory and contributed to the event where over 34,000 customers lost power. Household consumers and businesses were left without power on one of the coldest nights this year. That's not good enough."
The review identified several communication and process issues and found Transpower's tools and communication protocols had not been adequately tested for a national grid emergency.
The authority said the system operator's notices at the time did not give enough information for distributors to understand the wider context of the event.
It said the lack of clarity resulted in distributors and retailers not having enough information to ensure they communicated the extent of the issue with their customers.
This caused confusion among distributors as to whether notices issued by the system operator were calls to immediate action or forewarning of possible future action, resulting in some distributors being unsure about the action required, it said.
The Authority recommended Transpower further electricity sector readiness to respond to critical demand management incidents.
This included an annual industry-wide exercise to test processes, actions and communications and clarify responsibilities ahead of responding to a real emergency.
It said the August incident also highlighted serious issues with Transpower's load shed and restore (LSR) decision support tool, which was used to generate the demand allocation.
Transpower quickly suspended the use of the tool when it became apparent it was producing incorrect results, the Authority said.
"We recommend the system operator complete a thorough review of this tool before it is reinstated to ensure it meets the needs of the current power system," Stevenson-Wallace said.
The Authority said it was confident the steps taken by the system operator since the event, the recommendations highlighted in the report and any further recommendations would support an improved response in the future.
"The Authority takes any failure in the system very seriously. Transpower, as the system operator generally performs an excellent service on behalf of New Zealand consumers. The Authority has identified some matters for improvement and will work closely with Transpower to ensure the recommendations are implemented and safeguards are in place."
The Electricity Authority is doing a broader report into the blackouts, which will include the actions of generators.
Transpower said it accepted the Electricity Authority report.
Chief executive Alison Andrew says it has spent the past few weeks supporting the authority in its review, while also looking into its own communication with the industry.
She said Transpower had already made improvements, as acknowledged by the authority.
Transpower has two weeks to respond to the specifics of the report.