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Highlights • Gamma Knife efficacy for treating greater numbers of brain metastases is unclear. • Survival outcomes in 180 cases with a median 5.5 (range 1–47) lesions were examined. • Total tumor volume was more effective in predicting outcome than tumor number. • Cases with volumes >9.1 cm3 had the poorest median survival time of 5.2 months. • A tumor volume index is encouraged for predicting stereotactic treatment outcomes. Abstract Background and purpose To assess the response of the first cohort of patients treated with Gamma Knife radiosurgery in Australia. Materials and methods A prospectively collected cohort of 180 patients with intracranial metastases from different primaries was treated between August 2010 and July 2017. Survival was calculated using the Kaplan–Meier's method. Cox regression was used for multivariate analysis. Results Currently 141 patients (78.3%) have died of their disease. The median survival for the group as a whole was 9.2 months, with observed differences resulting from the volume of tumor burden (11.4 months for volumes <3.2 cm3 to 5.16 months for volume >9.1 cm3). Overall 2-year survival was 20.7%. Conclusion Results from the first Gamma Knife radiosurgery center in Australia showed that the treatment is feasible and effective, consistent with the international experience. For patients with larger numbers of intracranial metastases, the total volume of the intracranial burden may be of more significance in predicting outcomes. While there appeared to be a difference in survival by histologic origin, this could be related to concurrent systemic immunotherapy available for certain tumors. [ABSTRACT FROM AUTHOR]