If the ATO does not agree with the figures in your return, they may contact you to request further information. The ATO may ask you to provide support for the amount of income you have declared or the deductions you have claimed.
If you aren't able to provide the relevant supporting documentation (e.g. receipts) then the ATO may issue a 'Notice of Amended Assessment'. This will set out the ATO's view on your tax refundable/payable and it is then up to you to disprove their assessment.