Branch stacking is a term used in Australian politics to describe the act of recruiting or signing up members for a local branch of a political party for the principal purpose of influencing the outcome of internal preselection of candidates for public office, or of inordinately influencing the party's policy.
Allegations of such practices have become controversial in Australia after several inquiries or contests which received mainstream media attention. Most political parties now have clauses in their constitutions to allow "head office" intervention to resolve alleged stacking or other allegations of fraud, with penalties for those who engage in it. Branch stacking itself is legal under Australian law since it is an internal party matter, but some activities like providing false information to the Australian Electoral Commission, such as the numbers of members, can be prosecuted as fraud.
There are several ways that branch stacking may influence the way in which decisions are made within political parties. For example, a party faction may enrol many members who belong to the faction or agree to vote in the manner instructed by the faction's leaders.