ATO Secure mail attachment limits

Last checked 30 October 2026 against official sources

When you send documents to the Australian Taxation Office through Secure mail in ATO online services, the attachments have to meet a set of rules. This is a summary of what the ATO publishes, with links to the official pages below.

Who uses Secure mail

Secure mail is in Online services for business (under the Communication menu). The ATO's page on using ATO online services says sole traders with an active ABN can also send or access secure mail in Online services for individuals and sole traders, by going to My profile and selecting Communication, then Secure mail, and that the Online services for business guidance is also relevant there.

The attachment rules

The ATO's troubleshooting advice says supporting documentation attached to a secure mail message must:

• be a maximum of 6 files
• be less than 6 MB in size (per file)
• be in a suitable format: doc, .pdf, .rtf, .xls, .xml, .tif, .jpg, .zip, .bmp, .png, .gif, .mpp, .ppt, .docx, .dotx, .xlsx, .xltx, .pptx, .potx, .ppsx
• not be encrypted (for example, password protected)
• not contain macros
• not contain formulas (the ATO notes that copying and pasting from one spreadsheet to another may introduce unseen elements)
• not be an executable file (such as .exe, .app, .vb or .scr).

The ATO also asks for a separate secure mail message for each subject, and says a mailbox has a 500-message limit that includes sent items.

Example: super guarantee charge statements

On its super guarantee charge page, the ATO says you can attach up to 6 spreadsheets when lodging SGC statements by Secure mail, with each attachment having a maximum file size of 6 MB, and that the spreadsheet should be saved in .xls format.

How to fit your documents to these rules

• More than 6 documents? Combine related pages into one PDF with Merge PDF, or turn several photos of receipts into one file with Images to PDF. Number the pages with Add page numbers so the reader can refer to them.
• A file of 6 MB or more? Shrink a scanned PDF with Compress PDF, or split it into parts. Large phone photos can be reduced with Compress to 200 KB or a custom limit.
• iPhone HEIC photo? HEIC is not in the list; convert it to JPG.
• WebP image? WebP is not in the list either; use WebP to JPG.
• Password-protected PDF? Save an unprotected copy first — encrypted files are not accepted.

Privacy

Tax documents contain your TFN, income and bank details. FileKivo tools run entirely in your browser, so files you merge, compress or convert here are not uploaded to FileKivo. Only send documents to the ATO through ATO online services or other official channels.