2025-26 Supplementary SRESS for Planned Outages

The ISO has published its Final Decision to expand the circumstances in which Supplementary SRESS contracts may be used to include planned outages where their use is required to maintain adequate headroom levels.

Notice: Final Decision – 2025-26 Supplementary SRESS for Planned Outages

Background

On 28 November 2025 the ISO published its Final Decision for 2025-26 Essential System Services (ESS) following consultation on its Draft Decision under Rule 200(1). The ISO subsequently entered into Supplementary SRESS contracts with Horizon Power on 28 November 2025 and with APA DEWAP on 8 May 2026.

The Final Decision stated that Supplementary SRESS would be enabled only in limit circumstances and would not be used for planned outages other than islanding events. Under clause 3.14.2 of the Energy Balancing and Settlement (EBAS) Procedure, the NSP responsible for a planned network outage would be required to implement arrangements to meet the System Security Objective in Rule 162. Under clause 3.14.7 those arrangements may include additional ESS.

With two Supplementary SRESS contracts in place, and having considered operational experience since the 2025 Final Decision, the ISO decided to expand the use of those contracts to planned outages of network elements where required to ensure adequate headroom levels. This approach is consistent with the Pilbara Electricity Objective and the ESS Objectives.

The ISO consulted the consultation process outlined in Appendix 1 of the Rules and considered stakeholder submissions in making its decision, as published below.