A Director Penalty Notice is the ATO’s final warning. The 21-day clock starts...

https://wiki-nest.win/index.php/How_Accountants_Monitor_ATO_Liabilities_in_Real_Time_to_Prevent_DPNs

A Director Penalty Notice is the ATO’s final warning. The 21-day clock starts the moment it is issued, not when you open the mail. If your PAYG, GST, or SGC returns aren’t lodged, you face a lockdown DPN, making you personally liable for the debt