If you originally registered in Austria using a British passport, have not included your other nationality on your MeldebestätigungMeldezettel Austria requires everyone living in Austria (both Austrians and foreigners) to register their place(s) of residence with the local authorities. The confirmation of this registration is called a Meldebestätigung (EN: confirmation of registration), although it is still affectionately known to many by its former name (the Meldezettel). This is done at your Meldeamt in the Gemeinde or Bezirk you live in. You are required to do your registration within three days of arrival. It is important to do this also because your qualification for permanent residence starts to run from the point of your registration. or AnmeldebescheinigungAnmeldebescheinigung Every EU citizen who intends to stay longer than 3 months, must apply for the Anmeldebescheinigung within 4 months of first arrival in Austria. This is essentially a snap-shot of your status at that time, and confirms you have fulfilled the conditions to stay in Austria for longer. Your residence is still subject to certain rules. Note: British Citizens could only apply for the Anmeldebescheinigung until the end of the transition period (31.12.2020), you will need to update these accordingly so the authorities are aware of your other (and therefore EU) nationality, otherwise you will have to apply for the new Art 50 EUVAufenthaltstitel "Artikel 50 EUV" The Aufenthaltstitel "Art 50 EUV" is a residency title to be issued to British citizens from the end of the transition period. Its name is derived from Article 50 of the Treaty on European Union. The "Titel" refers to the document or card itself, and not the right, or permission that it confirms. card in 2021.
In addition, you should also inform both the ÖGK and Finanzamt – your employer may be able to assist in this regard through payroll accounting.