Introduction: Mepolizumab (Nucala) is a high-cost biologic where cost-effectiveness depends on patient continuation. While national uptake is increasing, it is unknown if growth patterns are uniform or suggest issues with program maturity and management across Australia. Aim: To investigate and compare long-term growth trends of Mepolizumab prescriptions in each Australian state/territory, using the rate (Slope) and stability (R²) of growth as proxies for market maturity and prescribing consistency. Methods: A retrospective analysis of monthly PBS prescription data (per 100,000 capita) over 30 months was performed. Simple Linear Regression was run for each state/territory to model growth rate (Slope) and "Goodness of Fit" (R²). A high R² value indicates a stable, predictable pattern, while a low R² suggests an erratic, immature market. Results: The analysis revealed two distinct prescribing models. "Mature Markets": Most states, including QLD (Slope=0.258, R²=0.91) and WA (Slope=0.252, R²=0.92), demonstrated steep, predictable, significant growth (all p < 0.001). This suggests a mature model with new patients steadily added to a stable, growing pool. "Immature Markets": In contrast, the ACT (Slope=0.157, R²=0.71), TAS (Slope=0.186, R²=0.73), and notably the NT (Slope=0.143, R²=0.44) showed flatter growth and weaker, more erratic trends (low R²), proving the linear trend is a poor fit. Conclusion: Uptake of Mepolizumab is not uniform. While most states show mature growth, prescribing in the ACT, TAS, and NT is erratic. This instability (low R²) is partially explained by low-volume markets, where small patient number changes create large statistical variance. This, combined with clinical factors like limited specialist access, "boom-and-bust" prescribing (especially NT), or inconsistent clinical awareness, results in slow, volatile uptake.