New Zealand, like many other countries around the world, is developing Government policies requiring open access to public data. The National Institute for Water and Atmospheric Research (NIWA) has been directed to make subtantial parts of its fisheries, climate, coastal, oceanic and freshwater data more publicly available for re-use. NIWA recognises that making such data available is of very limited value, if potential users do not have access to suitable tools to work with these data, ie: GIS applications. As part of its Open Data programme, NIWA's Fisheries and Environmental Centers have funded enhancements to an Open Source GIS application, QGIS, and made this application available as a free download, along with NIWA data. This approach enables the effective re-use of NIWA (and other agencies') environmental and spatial data by individuals and organisations who otherwise have little or no access to commercial GIS tools. This presentation discusses the value of Open Source (and Open Standards) to support Open Data initiatives, and NIWA's experiences along the way.