Global climate change is profoundly affecting precipitation patterns worldwide, including the exacerbation of aridity in dryland regions. Here, we examined the variability in rainfall stable hydrogen and oxygen isotope compositions (delta 18O and delta 2H) at five locations in subtropical northwest Australia over 10 years (2015-2024) to better understand the mechanisms driving precipitation and to identify moisture sources. We found that precipitation with low delta 2H and delta 18O was predominantly observed during months with high rainfall, confirming the significant impact of the "amount effect" at most sites. delta 2H and delta 18O were positively correlated with the stratiform fraction of total precipitation, which indicates the importance of sub-cloud evaporation losses during stratiform events. The mean sub-cloud evaporated fraction was estimated at similar to 30%; a back trajectory analysis revealed that up to 47% of rain moisture during the wet season was recycled from land evapotranspiration. Differences between arithmetic and volume-weighted monthly mean stable isotope compositions highlight the seasonal significance of small-volume rainfall events for monthly means. We propose a new "cut-off" approach to eliminate the disproportionate influence of a high proportion of low-volume rainfall on the stable isotope composition of monthly precipitation and Local Meteoric Water Lines.