Systems analysis allows quantitative empirical testing of models that exist in the study of public policy. Simulation and visualisation techniques can help policy makers to reduce uncertainties on the possible impacts of policies. This paper presents a new tool for systems modelling and simulation of policy problems to support the problem analysis, the design of policy options and impact assessment activities. The tool facilitates the cognitive activity of representing complex mental models using system dynamics simulation modelling by defining standards and a procedure for policy modelling. We propose a new policy-oriented problem structuring method (PSM), the 'labelled causal mapping'. The method supports scenario-based dynamic simulation and provides graphical representations of the involved actors, key variables, control flows and causal dependencies in a policy decision situation. A web-based tool prototype has been implemented in a Node.js environment. Two real policy problems are presented for demonstration of the use of the prototype.