The 'CO2SafeArrest' Joint Industry Project (JIP) was set up with the twin aims of: (1) investigating the fracture propagation and arrest characteristics of steel pipelines carrying anthropogenic carbon dioxide (CO2), and (2) investigating the dispersion of CO2 following its release into the atmosphere. The project involves two full-scale burst tests of 24-inch, X65 buried line pipes filled with a mixture of CO2 and nitrogen (N2). An overview of the CO2SafeArrest JIP and details of the fracture propagation and arrest investigation appear elsewhere in two companion papers. This paper presents the experimental investigation and computational fluid dynamics (CFD) simulations of the dispersion of CO2 following its explosive release into the atmosphere over the terrain at the test site in the first test. The setting up of the experiment and the CFD model is described in detail, including the representation of terrain topography and weather (wind) conditions, and the condition at the 'inlet to the dispersion domain'. The modelling was carried out prior to the actual event, and simulated the dispersion of the CO2 cloud for different wind speeds and directions. This analysis confirmed that the sensor layout set up to obtain spot measurements CO2 concentration over the terrain at the site was adequate. The predicted and experimental values of CO2 concentration at the nominated locations over the duration of the dispersion were found to be in good agreement. Results of this study are expected to be used in developing a generalized model for the dispersion of CO2 and for estimating the 'consequence distance' for such events. It is noted that this distance is necessarily a function of time due to the highly transient nature of the event.