This study investigates practices of Indonesian local government budgeting during the reform implementation of a performance based budgeting system as the result of the adoption of New Public Management (NPM) in government entities. Despite there being few successful stories about the reform, the study aims to provide an empirical understanding of particular forms of budgetary practices in the context of a developing country that might be different from experiences in developed nations. Additionally, there have been very few interpretive management and accounting studies, over the past two decades, that have focused on local government budgeting. Most such studies were conducted in developed countries (e.g Rosenberg et al., 1982; Jonsson, 1982; Seal, 2003; Goddard, 2004, 2005; Seal & Ball, 2011) and this current study is a response to this research lacuna. Grounded theory is adopted to develop a substantive grounded theory on budgeting practices observed in three Indonesian local governments: Regency of BK (RBK), Regency of PM (RPM) and City of SM (CSM). The central phenomenon of substantive grounded theory is labelled as managing the harmonising of interest. The substantive grounded theory explained the particular ways used by actors in implementing budgetary reform, in which actors also actively attempt to harmonise the coexistence of various interests. This central phenomenon of managing the harmonising of interests occurs when the existence of fear, the presence of distrust and struggling due to complexity result from the influence of central and provincial government through rules, regulations and policies. Managing the harmonising of interests was described through managing public legitimacy, utilising procedural based control, managing trust relationships and playing power games. The outcome of managing the harmonising of interests is recognised as a decoupling between rules and routines. Further, this study links the substantive grounded theory with the perspective of institutional theory, as a general theory in conceptualising the budgetary changes. The main finding of the study explains a framework managing the harmonising of interests that illustrates the complexity of budgetary and NPM reforms. The framework can be interpreted as a response both to the coexistence of competing institutional logics and conflicting institutional demands in the institutionalisation process of a new budgeting system. The contribution of this research relates to empirical findings that enrich budgeting research on NPM in developing countries, employs interpretive methodology and grounded theory, and provides incremental development of institutional logics theory in explaining organisational change in a specific context.