Adelaide is the capital city of South Australia, one of the driest states in the world. The city receives its potable water from a series of water catchments and the River Murray. Unfortunately, the current drought combined with other environmental factors is putting a strain oil the River and its water quality is slowly degrading. A potential solution to Adelaide's reliance on the River is desalination of various alternate water Sources. The initial step in selecting the appropriate desalination technology is to perform and order of magnitude Cost Study. A simple technique for generating order of magnitude cost estimates is the development of a series of cost correlations based oil a cost database. A cost database of 300+ desalination plants was collated. The cost database demonstrated that Current large-scale desalination plants are capable of producing water in the range of $0.50-$2.00/m(3), depending oil plant size. Capital cost correlations as a function of plant capacity have been developed for large scale MSF, MED, SWRO and BWRO plants. Unit product Cost (UPC) breakdowns were also developed for these technologies. Using the cost correlations and UPC breakdowns order of magnitude estimates were performed. For the case-study of metropolitan Adelaide, reverse osmosis (RO) is the cheapest technology for seawater desalination.