One of the things the brands need for payment is a tax invoice for marketing spend. We do the hard work around this for you by automating invoices on behalf of influencers. For us to be able to do that for Australian influencers, you do need an ABN.
It's completely free and very easy to get a personal ABN. You can get one here
If you aren't able to get an ABN for any reason, you can of course submit offers without payment but you will need it if you require payment for posts. You are also not required to have an ABN if you are not an Australian resident for tax purposes.