This paper addresses a major reform driven by the Australian Government requiring initial teacher education providers to implement a validated final year teaching performance assessment, commencing 2019. It is in this context of introducing a new high-stakes, culminating assessment of beginning teacher competence that the concept of assessment fidelity emerged as a significant research and practice issue within a national consortium of universities trialling the Graduate Teacher Performance Assessment (GTPA). A large corpus of recorded talk of experienced teacher educators collected during the 2017 trial of the GTPA was analysed to identify and examine these conditions as well as risks. The analysis drew on Gee's notion of "identity" for examining situated meaning, and Fairclough's concepts of discourse, sociocultural practice and materiality, understood as interrelated. Understanding risks to fidelity is foundational to establishing the upper limits of the validity of an assessment and thus the defensibility of any claims made in relation to a performance on the assessment. With TPAs being positioned as a gatekeeper to graduation and work as a teacher, it is important that there is confidence in the assessment to generate evidence of competence.