Energy Security Outlook

The Energy Security Outlook (ESO) is a monthly report that helps track how secure New Zealand’s electricity supply might be over the next two years. It uses two key tools, the Electricity Risk Curves (ERCs) and Simulated Storage Trajectories (SSTs), to show how likely it is that we’ll have enough electricity, especially during dry periods when hydro lakes are low.

For more information on how it works and why it matters, we have published an 'ESO 101' document that covers how ERCs and SSTs help forecast electricity risks, what triggers action when supply looks tight, and how extra hydro storage can be used if needed.

Energy Security Outlook 101

September 2025 Energy Security Outlook

Energy Security Outlook - September 2025.pdf (PDF 3.54 MB)

The national controlled hydro storage position has increased since last month's update and sits at 102% of the historic mean at 25 September (up from 77% at 25 August). South Island storage is lower at 99%.
The Electricity Risk Curves (ERCs) have lifted for most months in 2026 relative to the August update, predominantly due to a decrease in gas storage levels. There was little change to the risk curves for the remainder of 2025 when the curves are set by the Alert and Emergency floors reflecting energy held in contingent hydro storage.
No Simulated Storage Trajectories (SSTs) cross the Watch curve in 2025. 31 of the 93 SSTs cross the Watch curve in January-July 2026. This assumes the third Rankine unit retires in January 2026 and the market
supplements the existing coal stockpile at its maximum import capability to maintain increased thermal generation during low hydro inflows.
Looking ahead to winter 2026, electricity supply risks can be reduced through hydro storage management and ensuring sufficient backup thermal fuel and capacity to support increased thermal generation under
extended periods of low inflows.
In this month's scenario we look at the impact on the ERCs if the third Rankine unit were to remain available in 2026, which is currently awaiting Commerce Commission approval. This lowers the NZ Watch curve by up to 931 GWh and results in four SSTs briefly crossing the Watch curve in 2026.

The graphs below compare New Zealand and South Island controlled storage to the relevant Electricity Risk Status Curves.