It is illegal under the Workplace Relations Act to restrict freedom of association which sounds like what they are trying to do.

The Qld IR Act would prevent such behaviour as it is far more friendly to unions than the federal legislation. Section 101 of the Qld legislation states that a person eligible to become a member of an industrial organisation may become or remain a member.

Actually if you really are a casual there would be no contract signed as employment is on a daily basis. But in short, casuals can be union members.