Collective academic supervision (CAS) is a collective model for students' academic supervision to reduce their isolation and as a measure to establish a congenial culture and to develop networks with their peers. Most studies focus on the benefits of online CAS, leaving the pedagogical process and students' learning experiences understudied. This research examines the participation and learning experience of a cohort of Master of Education (MEd) students in online supervision that took place on a Moodle platform. This article reports a case study of Moodle-based CAS in Hong Kong that aims to train postgraduate students into teacher-researchers. A class of MEd students and their supervisors were observed, and their online dialogues were analysed. The bio-ecological student engagement model was used to explain the online supervision process. The results indicated that the students' learning was situated and embodied in the online social processes facilitated by peers' and supervisors' replies. The online interaction behaviours mainly included proposing questions or problems, providing information or solutions, and making comments. The findings have provided an exemplary case regarding the application of the online learning environment in supporting CAS and active research-based learning. The productive online CAS seems to benefit both teacher candidates and their supervisors by promoting the co-construction of the knowledge and skills of educational research, although more evidence is needed. Copyright © 2021 The Authors. Journal of Computer Assisted Learning published by John Wiley & Sons Ltd.