Among the most important decisions in water companies are planning the Maintenance-Rehabilitation Works (MRWs) of water pipelines. Since the MRWs are costly, it is essential to use the collective decisions of an expert team for planning these works. However, the effectiveness of these group decisions is not very clear and has been less considered in previous studies. Hence, the main objective of this research is to assess the effectiveness of these collective decisions. For this purpose, the MRWs of water pipes are determined based on two methods. In the first, the collective decision of experts is omitted, and in the second one, these decisions are considered. Finally, the results obtained from these methods are compared. The method used in this work for collective decisions is based on the nominal group technique. Furthermore, since there is deep uncertainty in water network data as well as hesitation in the group decisions, a qualitative (fuzzy) risk-based group decision-making model is developed in this research. The water pipelines studied in this work, as the case study, are addressed to pipes, which have been proposed by water companies in the six provinces of Iran. The MRWs of these pipes are determined, first, using the collective decision of 76 experts in these water companies; and then, without this group decision making. The results of comparing these methods indicated that group decisions do not have a significant effect on prioritizing water pipelines for the MRWs. While, in determining the renovation strategies of pipes, the viewpoints of experts could have a decisive effect on the results. Furthermore, it was found that increasing the number of decision criteria could lead to more realistic results; whereas, the number of assessed pipelines as well as the number of decision makers does not have an obvious effect on the results.