Assessment decision-making is an integral part of teacher practice. Issues related to its trustworthiness have always been a major area of concern, particularly variability and consistency of judgment amongst teachers. There has been extensive research on factors affecting variability, but little is understood about the cognitive processes that impact the trustworthiness of assessment. Even in an educational system like Australia, where teacher-based assessment in the mainstream schooling system is widespread, it has only been relatively recently that there have been initiatives to enhance the trustworthiness of teacher assessment of English as a second or additional language or dialect (EAL/D), but how teachers make their decisions in assessing student oral language development has not been documented. In this study, I explored this issue using the oral assessment tasks and protocols developed as part of the Victorian project, Tools to Enhance Assessment Literacy for Teachers of English as an Additional Language (TEAL). I adapted the materials and applied them in the context of EAL/D learning and teaching in New South Wales, aiming to (1) examine to what extent EAL/D teachers’ oral assessments were consistent, (2) explore factors influencing their assessments, and (3) identify characteristics of teacher decision- making. Employing a mixed-method research approach, this study involved twelve experienced NSW primary and secondary EAL/D teachers who participated in a survey, an assessment activity and a think-aloud protocol followed by individual interviews. The findings revealed that teachers were different from each other in the ways in which they came to their judgment decisions and in their perception of student development and that the differences were affected by factors related to teacher and student demographics and the characteristics of the assessment tasks. One result of this study was the development of a new framework to understand teacher decision- making processes with three different styles namely: (1) self-regulated assessment, (2) conflicted assessment, and (3) automated assessment. These decision-making styles provide a new lens for explaining variability in teachers’ judgement of student oral language development. Implications of the framework for assessment theory and practice, teacher development, policy articulation and future research are also discussed.