This article explores the role of teacher unions in shaping teacher professionalism discourses in New Zealand. Through an analysis of organisational documents and mainstream public media reports from recent decades, the article contends that tension and cooperation serve as defining characteristics in the relationship between teacher unions and governments, particularly concerning professional matters. While teacher unions generally strive to collaborate with governments on professional issues, an underlying sense of mistrust and tension remains deeply ingrained in their relationship. This article sheds light on this dynamic by examining the introduction and implementation of two significant education policies during the 2000s and 2010s: the National Certificate of Educational Achievement (NCEA) and the National Standards. While these two education policies are official achievement and assessment standards implemented in secondary and primary school education, respectively, the two main teacher unions responded differently. This divergence in stances on education standards policies presents an intriguing area for investigation. This article highlights the often-overlooked role of teacher unions in shaping professionalism discourses, particularly through their critical interactions with the government. It delves into the political dynamics and competing ideologies that influence the broader landscape of teacher professionalism.