The way in which individuals and organisations make decisions and the criteria used to define a "good" decision have been an important area of study for a major part of this century. Two aspects of a good decision are good process (how the decision is made) and good outcome (what acutally happens). Social decision-making affecting the environment may have long-term, large scale consequences that often cannot be accurately measured or predicted in advance. Environmental decision-making is characterised by uncertainty as to what will occur, what the impacts will be and what is the chance of occurrence. Decision support may be described as any "data, information, expertise and activities" that assists the decision-maker. Decision support systems combine decision analysis and information modules. A decision support system framework was applied to the management of Lake Ellesmere, a large brackish water lake in Canterbury, New Zealand. The lake is recognised as a wetland of international importance with high cultural and recreational values. Currently, there is limited understanding of ecological processes in the lake. There is also considerable institutional uncertainty. As a result of this combination of factors it was determined that the use of a "soft systems" learning approach to management would be of considerable benefit to present and future decisionmakers and managers.