While ERP systems can improve the efficiency, effectiveness and flexibility of business processes, there exists a relationship between post-implementation modifications and the optimisation of business processes. This relationship has not been adequately researched. By drawing on the post-implementation experience of an Australian oil company, we report a series of modifications that were conducted on their ERP system, and empirically classify them via propositions according to our proposed theory-driven model: ERP PIM. The model and empirical evidence together provide a theoretical foundation for research into the impact of ERP post-implementation modifications and their influence on business process optimisation. This is considered important for achieving competitive advantage. The applicability of our model as a useful managerial tool for analysing various modifications and understanding their impact on business process optimisation is offered.