Decision on 21 December 2025 under-frequency event

An under-frequency event occurred at approximately 5.21pm on 21 December 2025 when Contact Energy Limited’s Tauhara generator tripped, reducing North Island frequency to 49.225Hz.

The Electricity Authority’s final determination, under clause 8.61 of the Electricity Industry Participation Code 2010, is that Contact Energy Limited, as the owner of Tauhara TAB G1, was the causer of this under-frequency event.

Our reasons for this determination are:

• Contact Energy is the owner of Tauhara TAB G1
• Tauhara TAB G1 tripped due to activation of the turbine differential expansion protection
• The trip reduced North Island frequency to 49.225Hz, which meets the Code definition of an under-frequency event
• The System Operator’s causation report identified Contact Energy as the causer
• Contact Energy accepted the Authority’s draft determination that it was the causer.

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 $136,700.

Any participant substantially affected by this determination may dispute it by contacting registrar@electricityrulingspanel.nz by 5pm, 26 August 2026.

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