I found this badly translated site that might be of some help. I think Amtspartei basically means an offical party representing the public interest in legal proceedings involving competition law.

Der Bundeskartellanwalt
Aufgabe des Bundeskartellanwalts
Der Bundeskartellanwalt ist - neben der ebenfalls per 1.7.2002 eingerichteten und im Wirkungsbereich des Bundesministeriums für Wirtschaft und Arbeit angesiedelten Bundeswettbewerbsbehörde -"Amtspartei":
Damit hat er zum Zwecke der Wahrung der öffentlichen Interessen auch in solchen kartellgerichtlichen Verfahren Parteistellung, in denen er nicht Antragsteller ist.

Public Prosecutor in Cartel Matters
Functions
The function of the Federal Cartel Attorney is the representation of public interest in affairs of the competition law before the Cartel Court.
This does not affect only the cases of the cartel law in the narrower sense, but also cases of the abuse of a market dominating position or proceedings of amalgamation.

For the purpose to protect the public interest he has by this means also the position of a party in such proceedings before the Cartel Court where he is not the applicant. Contact
However petitions to the Cartel Court concerning the only intended notification of amalgamation should not be submitted to the Federal Cartel Attorney, but rather directly to the Federal Competition Authority because the Federal Cartel Attorney does not deal with such petitions himself but only refers them to the Federal Competition Authority.

http://www.bmj.gv.at/EN/justiz/content.php?nav=59