The rapid development of the Central West Orana Renewable Energy Zone (CWO REZ) in New South Wales requires robust and scalable approaches to assess system stability as high volumes of inverter‑based resources (IBR) connect to a weak and evolving network. This work presents results of refined frequency‑domain stability assessment methodology developed and applied within the CWO REZ program during the past year. The approach integrates perturbation‑based impedance scanning in EMT, frequency‑domain evaluation in positive and negative sequence-domain impedance matrices, and cross‑validation with large synchronous condenser (SynCon) models to verify consistency between transfer‑function, state‑space, and impedance‑based representations.

Specific focus has been the evaluation of the subharmonic range and investigation into sub‑synchronous and near‑50 Hz oscillatory modes without requiring full‑system EMT models. Key contributions include (i) a harmonised workflow for extracting plant and system impedance characteristics, (ii) demonstration that the methodology is applicable to both grid-forming and grid-following IBRs as well as SynCons, and (iii) insights into model granularity needs near 50 Hz to capture critical system modes.

The results illustrate that frequency‑domain analysis can complement—and in some cases replace—traditional small‑signal stability studies, providing a transparent and reproducible process for REZ‑scale integration challenges. The methodology offers a practical path forward for developers, OEMs, and system operators seeking consistent, data‑driven stability assessment as renewable penetration continues to grow.