23 October 2025 under-frequency event

We are seeking feedback from electricity market participants who may have been substantially affected by an under-frequency event on 23 October 2025.

On 23 October 2025, at approximately 5.42pm, circuit breaker 662 at Wairakei was closed, energising circuit TAB-WRK-1. Circuit breaker 662 (CB662) at Tauhara was then closed. Within seconds, the Tauhara geothermal unit (TAB G1) and Te Huka 3 geothermal unit (TAC G1) both tripped. The resulting imbalance between generation and demand caused the North Island frequency to fall to a minimum of 49.2Hz. Automatic reserves responded and rebalanced the power system, returning frequency to the normal operating band by 5.43pm.

The Authority’s draft determination is that the System Operator caused this under-frequency event.

We welcome feedback on our draft determination by 5pm, 31 August 2026 and will consider all submissions before making our final decision.