career path - Academic Progression via Promotion vs via Recruitment

I'm currently a postdoc at School A at a certain university in Australia. On another campus of the same university, there's a vacant assistant professor position at Shcool B. I was intending to apply for this position, but the HR is telling me that I cannot and that my application should go through the academic promotion procedures.

Is this fair under the equal opportunity principles? I checked the policies available online in this regard but didn't find any explicit interpretation of cases like mine. Why should be disadvantaged just because I'm already employed by the university?

Edit: The academic promotion round takes place once a year in March. So I'll be missing out on this opportunity if I wait till next year.