It has been commonly believed that public opinion influences public policies in democratic countries. However, there has been a theoretical debate whether public opinion plays a major or minor role in the policy making process. Moreover, little research has been done on how public opinion plays a role in the policy making process, especially on environmental legislative processes. This study investigates the mechanism of the involvement of public opinion in the legislative process in New Zealand, focusing on a single case study, the Biosecurity Act 1993 (BSA). Because the BSA is regarded as the world’s most comprehensive legislation dealing with pest issues, it was assumed that if the role of public opinion was identified, it would contribute to the development of public involvement in biosecurity policy making in other democratic countries. The role of public opinion in the legislative process of the BSA was evaluated by considering political and social contexts. Because there are few methods developed in order to evaluate the role of public opinion in a particular policy or legislation, qualitative and dual-disciplinary (political science and sociology) approaches were taken. A conceptual framework was made combining the conpepts of the Policy Topography Model (PTM), the Advocacy Coalition Framework (ACF) and the Amplification Model (AM). Secondary data (books, journal artucles, newspapers, websites and government documents) and primary data (public submissions) were collected and examined as data. The research findings showed that there was little public opinion involved in the legislative process of the BSA. The reason for the low involvement was considered to be because of the issue complexity, its low legislative priority within government, “undemocratic” political situation, and the lack of social movements and public perception towards the bill. Now, 20 years has passed since the BSA was enacted. The BSA is now widely recognised by the public as an environmental legislation which manages the pests in New Zealand, although it was recognised as agricultural legislation when it was enacted. This was discussed with the development of democratic condition and social awareness in New Zealand. Ultimately, this research will provide insights into how public opinion can be best involved in the environmental legislative process by developing the conceptual framework.