so I have safari 14.1.2 - do I have to do anything about the security warning from the Australian Cyber Security Hotline ?

It would help if you advise in you question the alert you are asking about. I presume it is this update from 14/09/21.

The only thing you need to do, and do regularly, is keep your devices up to date. Apple released some updates recently. The alert from the agency is nothing more than advice to Australians that Apple has patched some identified vulnerabilities. This happens regularly. Every time your device tells you there is an update to apply, run the update. In the case of Safari 14.2.1 you should have updated to MacOS 11.6 by now.