If you want to act on behalf of an adult in electronic services, they need to provide you a mandate for transactions.

If the person on whose behalf you want to act cannot use electronic services, they can grant mandates and approve mandate requests with assistance. Read more about assisted authorisation.
Instructions for using a mandate are available at How do I act on behalf of another party with a mandate?.
If you want to manage the affairs of an adult as their attorney or guardian, more information is available on the following pages: