Australia has large deposits of critical minerals that are mainly exported for processing in other countries. Building a strong domestic pyrometallurgical and hydrometallurgical infrastructure is essential for transforming these raw ores into high-purity products. This step will help Australia improve its position in the global energy transition. By processing these resources locally, the country can secure its own supply chains and promote industrial independence. The recent shift of the Kalgoorlie Nickel Smelter to care and maintenance highlights a strategic vulnerability in Australia’s midstream processing ability. To ensure national independence and gain the full economic benefits of our resources, it is crucial to improve the technical frameworks that support our metallurgical sites. A primary barrier to the processing polymetallic concentrates domestically is managing harmful impurities, especially arsenic (As). This research addresses this issue by re-optimisation of the Ni Fe-Cu-S-As system from a thermodynamic perspective. The development of a high-fidelity predictive model for high-temperature phase equilibria facilitates the efficient processing of complex, arsenic rich sulfide ores that would otherwise present significant economic or environmental challenges. By developing a reliable predictive model for phase equilibria at high temperatures, this study enables more efficient treatment of complex sulfide ores. This precision is key to cleanly separating valuable metals from waste, helping to overcome the economic and environmental hurdles that typically stall the processing of these deposits. By combining equilibration-quenching experiments with precise Electron Probe Micro-Analysis (EPMA), this work provides the fundamental data required to improve furnace settings at major Australian sites, including Olympic Dam, Mount Isa, and the Kwinana Refinery. Crucially, this research provides the technical foundation and offers options to support the potential reopening and long-term sustainability of important facilities like the Kalgoorlie Nickel Smelter. By improving our understanding of impurity behaviour and phase equilibria, this work ensures that when market conditions improve, Australia has the necessary metallurgical tools process various sulfide deposits locally. This will help secure a self-sufficient and sustainable supply of metals for Australian industry and infrastructure.