The 21-day DPN clock starts on the issue date, not when it hits your desk....

https://www.lawyersweekly.com.au/sme-law/44139-what-solicitors-need-to-know-when-a-client-receives-a-director-penalty-notice

The 21-day DPN clock starts on the issue date, not when it hits your desk. Distinguishing between lockdown and non-lockdown notices is essential for effective client protection. This guide covers the basics, so you can triage fast and keep options open.