This study aims to understand the improvement in building performance vis-à-vis a rise in green building certification levels. It investigated the National Australian Built Environment Rating System (NABERS), one of the few government initiatives that rates energy performance, emissions, water consumption, and indoor environment quality (IEQ) separately based on their corresponding post-occupancy data. A set of 2657 certified records of office buildings and their performance data acquired from the NABERS database were analysed. Performance indicators for the analysis were energy use intensity (EUI), emission intensity (EMI), water consumption intensity (WCI), and indoor environment quality score (IEQS). The results revealed a linear relationship between each indicator's performance data and certification level—a one-level rise reduced EUI, EMI, and WCI by 184.45 MJ/m2, 41.94 kgCO2-eq/m2, and 0.24 t/m2, respectively, per year on average. On the other hand, further rise in the level made the reduction less significant. The reduction also varied from the baseline, which is related to climatic and socioeconomic factors. This study also found that rise in IEQ and energy certification levels correspondingly raised the ratio of IEQS to EUI and EMI, indicating quality/load. Thus, compared with other major green building rating systems, NABERS could differentiate building performance more effectively using different certification levels. This study addresses the baseline issue as well and provides suggestions to improve NABERS and other green building rating systems.