It can be said that a geomembrane liner system is only as strong as its weakest weld. This statement is particularly true for geosynthetic lined dams, which require a large number of discrete geomembrane panels to be joined together to create a low permeable barrier. In recent years dam owners have recognised the importance of answering the question “how long is the service life of the geomembrane?” This is reflected by their readiness to invest in testing a variety of different types of geomembranes to examine properties related to durability and degradation; an exercise required to ensure suitable materials are procured for the project-specific application. However, premature failure of the geosynthetic lining system can also arise from weak elements which are built into the system during construction. This paper presents the results of an investigation carried out on a geomembrane lined dam in Australia which was temporarily decommissioned due to elevated leakage rates and observed development of geomembrane ‘whales’, after it was in service for only 4 years. The investigation identified a number of geomembrane cracks (or splits) in the vicinity of the whales, located parallel to the outside edges of fusion wedge weld tracks where adjoining geomembrane panels were seamed. The heat affected zone of a weld adjacent to a 400 mm long crack was analysed in the laboratory to examine a number of geomembrane properties, including dimensional changes in the weld profile, Standard Oxidative Induction Time (Std-OIT), cyclic flexural testing to failure, and Weld Interface Light Interface Microscopy Examination (WI-LTME). Testing was also performed on an intact weld, which is referred to as the control sample. The testing confirmed the heat affected zone adjacent to the crack was more susceptible to brittle stress cracking compared to the control sample, and exhibited raised crystallinity which is indicative of higher than industry standard welding temperatures and slower cooling. It is hypothesised that failure of the geomembrane occurred at the edges of fusion wedge weld tracks due to the combined effects of 1) raised crystallinity from unacceptably high welding temperatures; 2) geometric stress concentrating features along the weld profile, and 3) variations in ambient temperatures causing whales to ‘breath’, inducing a flexing action on the edge of welds. The geomembrane cracks were repaired in situ and the impact on leakage rates was assessed after the dam was recommissioned. The lessons learnt from the investigation are presented in this paper. This paper also demonstrates that the use of Action Leakage Rates (ALRs) are an effective tool for dam operators to assess the performance of the geosynthetic lining system, by comparing the monitored leakage rates with the calculated ALR, which then informs appropriate remedial actions to be taken in the event of the ALR being exceeded.