Question: My client was claiming part year threshold as he left Australian in January 2021. He has a ton withholding from his interest income, but the system does deduct it from his tax payable. 

Answer: Make sure "Australian Resident" must be marked as "NO" since the taxpayer is not a resident for the whole financial year.

Note, both ‘Gross interest’ and ‘TFN amounts withheld’ are not included in amount payable when person was a foreign resident when received or was credited with the interest.

https://www.ato.gov.au/Individuals/Tax-return/2021/Tax-return/Income-questions-1-12/10-Gross-interest-2021/