There is a field “Additional Information for the ATO (Other Attachments Schedule)”, available via Individual Tax Return, Partnership Tax Return, and Company Tax Return.
You may only add a text, but cannot attach additional documents,
Note: this is visible to ATO.
For Trust Tax Return,
Select Beneficiaries, scroll via item 61 Schedule A
NOTE: Attaching additional file or supporting documents are not uploaded with the tax return but should be kept on file in case the tax authority requests them during an audit or review.