Research have revealed that the accounting profession engage multi-disciplinary teams in the assurance of sustainability reports. However, how such teams are composed or the values that these teams bring to the assurance engagement process, and how reliance is established on the work performed by such teams has been left uncovered. This research sought to fill this gap, and hence, contributed to providing an understanding surrounding this issue. Semi-structured interviews with auditors from the Big Four Audit firms, engaged with performing sustainability assurance was conducted. The findings revealed that the engagement of multi-disciplinary teams, comprising both financial accountants and sustainability specialists from various disciplines, was necessary in ensuring quality assurance work performance. Firstly, the in-depth knowledge of sustainability subject matters possessed by the sustainability experts, enabled the questioning of the correctness of sustainability items reported by the clients. Secondly, sustainability experts were valuable in the planning stage of the engagement, in identifying client risk areas. Thirdly, sustainability expert involvement, enabled the winning of work contracts. Additionally, the engagement of multi-disciplinary teams was revealed to be valuable to the client firms. Firstly, the sustainability experts assisted clients in identifying the most valuable issues to report to their stakeholders. Secondly, due to the expert’s knowledge and experience, it enabled them to inform clients on high reporting levels. Finally, sustainability experts provided clients with valuable feedback that aids the improvement of the client processes, and the management of risks. The establishment of reliance on the work performed by the teams, requiring an accountant to signoff every engagement work performed by the team, as required by the applicable assurance standards, was partially supported by this study. Sustainability experts, who were non-accountants, was revealed to be engaged in this signing off ritual, hence, leaving room for the questioning of such assurance engagements signed off by sustainability experts.