The 14 and 28 Day rules for Employers relating to Tax File Numbers.
If an employee does not give you a valid Tax file number declaration within 14 days of starting an employer/employee relationship, you must complete a Tax file number declaration with all available details of the employee and send it to us.
If the employee has not given you their TFN within 28 days, you must withhold 49% from any payment you make to a resident employee and 47% from a foreign resident employee (ignoring any cents) unless we tell you not to.
ATO: Australian Tax Office
https://www.ato.gov.au/Rates/Fortnightly-tax-table/?page=2