Abstract Soil gas baseline and assurance monitoring was performed over the CO2CRC Otway Project demonstration site for six years (2007–2012). The main objective of the monitoring was assurance monitoring, to demonstrate to regulators that there were no adverse environmental impacts on soils. The use of the method for leakage detection was also of research interest. Soil gas sampling was carried out annually over a defined grid, in preparation for and during the experimental storage of 65,000tonnes of carbon dioxide (CO2) into the Waarre C Formation at a depth of about 2km. CO2, methane, oxygen and nitrogen were the main gases analysed. Advanced analytics included helium and δCCO213 measurements. Baseline monitoring allowed us to develop an understanding of the local variability in soil gas CO2 concentrations in anticipation of the CO2 injection. Interpretation of the concentrations of CO2 in terms of possible leakage rates (fluxes) is not possible without more detailed models and measurements of the possible transport mechanisms of CO2 to and through the vadose zone. However, the comparison of post-injection with baseline data, the examination of fixed gas relationships and C/13CCO212 isotopes showed that most CO2 in the soil was of biogenic origin. A deep subsurface source, which would indicate deep gas migration to the surface, was not apparent. This shows that there is little impact of the injected CO2 on the local ecosystem and, in conjunction with other monitoring data, helps to satisfy our regulatory requirements. Our work is a case study of a practical soil gas monitoring programme and illustrates both the utility and the limitations of the technique.