Decision on 21 April 2025 under-frequency event

An under-frequency event occurred at 4.43pm on 21 April 2025 when Huntly Unit 4 tripped offline reducing North Island frequency to 49.16 Hz.

Our final determination under clause 8.61 of the Electricity Industry Participation Code 2010 (Code) is that Genesis Energy Limited, as the generator that owns Huntly Unit 4, was the causer of this under-frequency event.

Our reasons for this determination are:

• Genesis owns and operates Huntly Unit 4
• Huntly Unit 4 tripped offline, and frequency in the North Island dropped to 49.16 Hz
• the System Operator investigated the event and identified Genesis to be the causer
• the Authority is not aware of any other acts or omissions by; or property of another participant which contributed to the event.

The clearing manager will rebate the event charges to the relevant participants, in accordance with clause 8.65 of the Code. The event charge calculated in the System Operator’s report to the Authority is $217,300.

Any participant who is substantially affected by this determination may dispute the determination by contacting registrar@electricityrulingspanel.nz by 5pm, 17 October 2025

The Authority did not consult on this event as it is no longer required to publicly consult when the alleged causer accepts it was the causer of the event (clause 8.61(2A) of the Code).

The System Operator's report into the event.