As the digitisation of public administration continues, the analysis of the environmental impact of platforms providing spatial data is becoming increasingly important. Public geoportals, while serving a crucial role in information dissemination and decision support, are rarely evaluated in terms of their CO₂ emissions or their compliance with environmentally sustainable design principles. In this study, ten selected public geoportals were examined using a comprehensive set of auditing tools, including Ecograder, Website Carbon Calculator, Digital Beacon, Kastor Green, Lighthouse, RankMatch, and the Green Web Evaluator AI agent. The evaluation focused on multiple dimensions, such as page performance, hosting solutions, resource size, accessibility, and the degree of SEO optimisation. The results revealed substantial variation in carbon emissions, ranging from 0.17 to 2.95 g CO₂ per page load, and highlighted numerous shortcomings in technical optimisation and compliance with WCAG accessibility standards. A strong correlation was observed between page size (MB) and carbon emissions, as well as a negative impact of redirect mechanisms on both performance and carbon footprint. These findings indicate the lack of a standardised model for building geoportals that balance functionality with environmental efficiency. The study concludes with a set of actionable recommendations aimed at supporting public institutions in developing geoportals that are more energy-efficient, accessible, and environmentally sustainable.