Yes. One person can hold both roles
There's nothing stopping a company director from also serving as company secretary. Australian law places no restriction on combining these two roles in the same person.
If a director wants to take on the secretary role as well, they're free to appoint themselves. As long as the appointment is properly registered with ASIC, there's no issue and no penalty for doing so.
Given the overlap in duties between the two positions, it's a common and workable arrangement; the important thing is that the person genuinely carries out the obligations attached to each role, not just one.
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Learn more: Can a Company Secretary Also Be a Director?