What is unknown about commercial lobbying is far greater than what is known. These omissions distort our understanding of the extent and nature of business influence on politics. Especially when businesses engage in practices that harm health, it is crucial for public health advocates to understand corporate lobbying to counter its influence. Our study proceeded in three phases. First, based on an international audit, we developed a list of the categories of information about lobbying that could be disclosed under four groups (lobby firms, lobbyists, organizations and activities) and benchmarked Australian lobbyist registers against this list. Second, we manually extracted data from lobbyist registers in eight jurisdictions, cleaned the data and created a relational model for analysis. Finally, we classified a sample of organizations as public health organizations or harmful industries to compare their activities. We identified 61 possible categories of information about lobbying in international lobbyist registers. When applied to Australian lobbyist registers, Queensland covered the widest range of categories (13, 21%), though many lacked detail and completeness. Australian lobbyist registers provided data on 462 third-party lobby firms across Australia, currently employing 1036 lobbyists and representing 4101 organizations. Several of these represented harmful industries, with gambling interests hiring the most third-party lobby firms. Ultimately, Australian lobbyist registers do not provide enough information to understand the full extent of lobbying activities taking place. Political transparency is important for public health actors to be able to monitor corporate political activity and to protect policy-making from vested interests.