This paper considers an incentive mechanism coupled with a set of collaborative rules in a decentralised collaborative transport network (CTN), taking the Physical Internet as an example. The goal of the proposed mechanism is to increase the efficiency, effectiveness, and sustainability of the network without decreasing the individual profit of the independent carriers. A multi-agent simulation model was developed to evaluate the performance of the proposed mechanism and rules and to analyse the impacts on the overall performance of the decentralised CTN. Moreover, two significant factors were identified and studied: network and market characteristics (e.g., demand to supply ratio), and competition between carriers. A baseline scenario with no collaboration was also simulated for comparison. The results indicate that collaborative rules are advantageous for all market types regardless of the competition in the network. This paper is among the first to investigate collaborative mechanisms and rules for decentralised CTN, especially with regard to sustainability issues. It also provides an effective methodology for designing mechanisms and rules in decentralised CTN, as well as for assessing performance.