When lobbyists lobby public office holders you should think of the latter as really meaning any public servant, including you, your staff, senators and their staff. When they do, they need to register. But there's a subset of that group, designated public office holders, which includes you, members of Parliament, senators, and obviously ministers—they're members of Parliament—and their staff. When they leave office, they're subject to a five-year prohibition on lobbying.
The exemptions under the act are extremely limited. They're really for people who have been there for a short period—students, individuals who really had only administrative duties. The exemptions are not for those who had any decision-making power. Students in ministers' offices are designated public office holders, so they will often be the ones asking for an exemption when they leave office.