PurposeThis article explores officer use of suspicion before informal police-citizen encounters as a method to further understand police officer decision-making. There is a body of research focused on officer decision-making before formal “stop and search” encounters, yet, while the more informal “stop and chat” encounters are ubiquitous, they are a comparatively under-researched part of policework.Design/methodology/approachThe research takes an ethnographic approach to explore police decision-making. It used participant observation (800 h over 93 patrol shifts) of front-line first response officers from New Zealand (n = 45) and South Australia (n = 48). Field observations were complemented with informal discussion in the field and 27 semi-structured interviews.FindingsIt reveals that officers applied three situational “tests” to assess the circumstances or actions observed before initiating an informal encounter. Officers then weighed up whether the circumstances were harmful, contrary to law, or socially acceptable to determine the necessity of initiating a police-citizen encounter. This process is conceived as suspicioning: deciding whether circumstances appear prima facie suspicious, how an officer goes about collecting more information to corroborate suspicion to ultimately inform a course of action.Originality/valueThe findings present a new perspective to understanding how and why police officers decide to initiate encounters with members of the public. Moreover, as the first ethnographic cross-national research of officers from New Zealand and South Australia, it provides a rare comparative glimpse of Antipodean policing.