[Australian Regulatory Requirement]
If Clients do not provide a TFN and they are not exempt from withholding tax (WHT) (like a pensioner), WHT is charged when interest is applied, if the interest income for the financial year projects to exceed AUD120.
There are more rules depending on age group. For details see linked document.
Note: Australian financial year is e.g. June 2018 -> July 2019.