"Senior and pensioner tax offset" can be found via  "Estimate" tab.

Make sure birthdate is correct, or adjust it via "Settings>General"

If you need to update the birthdate, make sure that the form is not lock/completed. (you can find this at top right of the form, click the lock icon)

Note: In order to claim senior tax offset of the spouse, use the option "Eligible spouse lived together" via "Offset" tab

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ATO SAPTO calculator