Decision on 23 October 2025 under-frequency event

An under-frequency event occurred at about 10.12am on 23 October 2025, when HVDC Pole 3 tripped offline and the frequency in the North Island fell to 49.02Hz.

Our final determination under clause 8.61 of the Electricity Industry Participation Code is that Transpower, as the grid owner, was the causer of this event.

Our reasons for this determination are:

• the grid owner owns and operates the HVDC
• HVDC Pole 3 tripped, and the frequency in the North Island dropped to 49.02Hz
• the System Operator investigated the event and found the grid owner to be the causer
• the grid owner accepted it was the causer of the event
• 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 charge to the relevant participants, in accordance with clause 8.65 of the Code.

Any participant who is substantially affected by this determination may dispute the determination by contacting registrar@electricityrulingspanel.nz by 5pm, Wednesday 4 March 2026.

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).