Soil organic carbon (SOC) is an indicator of soil health, as it exercises vital roles as a nutrient source as well as a contributor to a favourable soil structural conditions. It is well documented that cultivation and cropping of soil under native vegetation may lead to reduction in SOC. Estimates of 54% and 60% loss of SOC from initial levels have been reported for cotton and cereal based farming systems in the cropping region of north-western NSW. These estimations are useful in terms of gauging SOC levels under past management practices. However, of greater interest is the need to know how and at what rate SOC had changed over time as well predicting these changes into the future under existing and improved management practices. The Rothamsted Carbon Model (RothC) is a tool that facilitates the estimation of SOC changes in the past and their prediction into the future.In his study, we used soil profile data from the Australian Cotton Soil Database (ACSD) (Odeh et al. 2004) as inputs for a modified version of the RothC model to simulate SOC changes under existing management practices in a 3000km2 study area, west of Narrabri, NSW. Based on actual measurements performed in 1996, modelled estimates of SOC were considered reseanable. Estimates for 'continuous cotton' systems performed best (RMSE=0.19%), followed by 'cotton-legume rotation', 'cotton-grain rotation', 'native vegetation' and 'continuous grain' systems (RMSE= 0.24, 0.31, 0.35, 0.40% respectively). We then used the model outputs to interpolate onto a dense grid of the study area, the change in SOC from initial levels to 2003, using various ancillary data. We concluded that, in areas of irrigated cotton, there has been between 45-53% reduction in SOC from initial levels. To improve upon the models predictive ability, adjustments to some model inputs, verification of initial levels of SOC, determination of land clearing events and a harmonious land use classification, compatible to the ACSD are some means in which to do so. Overall, the base methodology developed during this exercise could be extended to other areas detailed by the ACSD. Value adding to this data through estimation of SOC fluxes under modified management systems and future climate scenarios are likely possibilities in future studies.