Purpose – The purpose of this paper is to explore the adoption of a time-based activity-based costing (ABC) information system in resource planning (ABP) in the Crown Prosecution Service (CPS) of England and Wales, a public service organisation in the United Kingdom, for the period of 2000-2005. The aims of the study are to provide a technical review of the ABP application and to explore roles of the ABP system in achieving a “fairer” internal resource allocation and an improved understanding of business processes. Design/methodology/approach – The CPS's experience of adopting an ABP system is explained through the use of a case study involving interviews, observations, work shadowing and archival data over a period of six years, 2000-2005. The longitudinal nature of the study has enabled the researchers to align the outcome of ABP adoption with concurrent organisational changes. Findings – The study reveals that the growing understanding of the intrinsic links between business processes and the ABP information and its rationale, which remains unchanged throughout various phases of organisational changes, has promoted a sense of stability amongst CPS staff members. Practical implications – This study provides an in-depth understanding of the practical use of ABP and its evolving roles in the face of the changing organisational environment. Originality/value – This research discusses the prerequisite of the ABP system, a time-based ABC system, and the evolving roles of ABP from a cost-effective planning tool to a stability mechanism in face of constant organisational changes. This experience is invaluable to companies and practitioners seeking to implement a cost-effective planning tool.