In a context of system-wide changes to mathematics assessment, primary (elementary) school teachers were brought together to develop a package of exemplary assessment tasks, rich in potential to provide data on students' mathematical understanding and knowledge, and relating to a new assessment framework. Through involvement in writing, the teachers were able to guide their own professional development and consequently improve their teaching and assessment practices. The writing team situation provided teachers with support, feedback, and opportunities for reflection; as well as assisting them to gain confidence in using and understanding the assessment framework. This paper reports the teachers" reactions to the collaborative writing process and evaluates the effectiveness of collaborative developments, of this form, as professional development. The teachers' responses to the program indicate that it was effective in improving confidence, knowledge and understanding with respect to mathematics teaching, learning and assessment because it provided opportunities for support, feedback and reflection on trials of ideas in classrooms.