Background: We are in a period of history where natural disasters are increasing in both frequency and severity. They are having widespread impacts on communities, especially on vulnerable communities, those most affected who have the least ability to prepare or respond to a disaster. The ability to assemble and effectively manage Interagency Emergency Response Teams (IERTs) is critical to navigating the complexity and chaos found immediately following disasters. These teams play a crucial role in the multi-sectoral, multi-agency, multi-disciplinary, and inter-organisational response and are vital to ensuring the safety and well-being of vulnerable populations such as the young, aged, and socially and medically disadvantaged in disasters. Communication is key to the smooth operation of these teams. Most studies of the communication in IERTs during a disaster have been focussed at a macro-level of examining larger scale patterns and trends within organisations. Rarely found are micro-level analyses of interpersonal communication at the critical interfaces between collaborating agencies. This study set out to understand the experiences of those working at the interagency interfaces in an IERT set up by the Canterbury District Health Board to respond to the needs of the vulnerable people in the aftermath of the destructive earthquakes that hit Canterbury, New Zealand, in 2010-11. The aim of the study was to gain insights about the complexities of interpersonal communication (micro-level) involved in interagency response coordination and to generate an improved understanding into what stabilises the interagency communication interfaces between those agencies responding to a major disaster. Methods: A qualitative case study research design was employed to investigate how interagency communication interfaces were stabilised at the micro-level (“the case”) in the aftermath of the destructive earthquakes that hit Canterbury in 2010-11 (“the context”). Participant recruitment was undertaken by mapping which agencies were involved within the IERT and approaching representatives from each of these agencies. Data was collected via individual interviews using a semi-structured interview guide and was based on the “Critical Incident Technique”. Subsequently, data was transcribed verbatim and subjected to inductive analysis. This was underpinned theoretically by Weick’s “Interpretive Approach” and supported by Nvivo qualitative data analysis software. Results: 19 participants were interviewed in this study. Out of the inductive analysis emerged two primary themes, each with several sub-factors. The first major theme was destabilising/disruptive factors of interagency communication with five sub-factors, a) conflicting role mandates, b) rigid command structures, c) disruption of established communication structures, d) lack of shared language and understanding, and e) situational awareness disruption. The second major theme stabilising/steadying factors in interagency communication had four sub-factors, a) the establishment of the IERT, b) emergent novel communication strategies, c) establishment of a liaison role and d) pre-existing networks and relationships. Finally, there was a third sub-level identified during inductive analysis, where sub-factors from both primary themes were noted to be uniquely interconnected by emergent “consequences” arising out of the disaster context. Finally, findings were synthesised into a conceptual “Model of Interagency Communication at the Micro-level” based on this case study of the Canterbury earthquake disaster response. Discussion: The three key dimensions of The People, The Connections and The Improvisations served as a framework for the discussion of what stabilises interagency communication interfaces in a major disaster. The People were key to stabilising the interagency interfaces through functioning as a flexible conduit, guiding and navigating communication at the interagency interfaces and improving situational awareness. The Connections provided the collective competence, shared decision-making and prior established relationships that stabilised the micro-level communication at interagency interfaces. And finally, The Improvisations i.e., novel ideas and inventiveness that emerge out of rapidly changing post-disaster environments, also contributed to stabilisation of micro-level communication flows across interagency interfaces in the disaster response. “Command and control” hierarchical structures do provide clear processes and structures for teams working in disasters to follow. However, improvisations and novel solutions are also needed and often emerge from first responders (who are best placed to assess the evolving needs in a disaster where there is a high degree of uncertainty). Conclusion: This study highlights the value of incorporating an interface perspective into any study that seeks to understand the processes of IERTs during disaster responses. It also strengthens the requirement for disaster management frameworks to formally plan for and to allow for the adaptive responsiveness of local teams on the ground, and legitimise and recognise the improvisations of those in the role of emergent boundary spanners in a disaster response. This needs to be in addition to existing formal disaster response mechanisms. This study provides a new conceptual model that can be used to guide future case studies exploring stability at the interfaces of other IERTs and highlights the centrality of communication in the experiences of members of teams in the aftermath of a disaster. Utilising these new perspectives on stabilising communication at the interagency interfaces in disaster responses will have practical implications in the future to better serve the needs of vulnerable people who are at greatest risk of adverse outcomes in a disaster.