Hello there, @2ezlouisy-lw-gma.
You can add your Australian Business Number or ABN in your sales forms using the QuickBooks Online Custom Form Style Feature. I'd be happy to show you how:
I've also included some screenshots for your additional reference:
To know more about this topic, you can check out this article: Customise invoices, quotes, and sales receipts in QuickBooks Online.
I've added this article for your future task whenever you'd like to bring a finished sales template to your QuickBooks Online company: Import custom form styles for invoices or estimates.
Feel free to comment below if you have other questions. I'd be happy to answer them. Take care always.