Can I do work on a subclass 408 visa that is different from my approved activity?

The Temporary Activity visa authorises you to participate only in the activity described in your application and supported by your sponsor or inviting organisation.

That means you cannot use the 408 as general permission to work in any job. Doing unrelated paid work can breach your conditions and jeopardise your visa. If your plans change or you want to undertake a different activity, you generally need a new or different visa rather than simply switching what you do.