An apostille is a short-form legalisation of documents currently used for international legal relations by approx. 100 countries, including Austria. The format of an apostille is standardised and must not be changed. If a country is a signatory to the Hague Convention Abolishing the Requirement for Legalisation for Foreign Public Documents it suffices to affix an apostille by the authority provided for in Article 6 of the Convention.  
For a list of competent foreign authorities for countries having signed the Convention see: http://www.hcch.net/index_en.php?act=conventions.authorities&cid=41