Hey, I just got a reply:
Other request documents: Employment contract or employment certificate in their country or professional portfolio showcasing digital nomad, remote worker, foreign talent or freelancer status
I have my own abn (Australian business number) for 15 years, running a small web design agency. In my cover letter I gave them a link to my business website (portfolio is at the bottom). I also attached some recent invoices showing the work completed.
Any help would be appreciated, thanks.