The post-seismic failures can cause serious damages to many geotechnical vulnerable sites within a considerable short duration. Most of these failures occur even after the ceasing of driving forces but can lead to a significant loss of life and property. Plastic waste is a non-degradable waste and its utilisation in any mitigation project will lead to a successful approach towards sustainability. Therefore, this study facilitates the sustainable utilisation of polyethylene terephthalate (PET) fibres in mitigating the post-liquefaction of fine sand. Several cyclic triaxial tests were performed on both the reinforced and unreinforced specimens of fine sand. A total of three different fibre contents (0.2%, 0.4% and 0.6%) were used in the study for reinforcement purposes. Two different relative densities of 30% and 50% were considered for a better observation of the density effect. The presence of fibre was found to be more effective in the medium-dense specimens due to a better fibre-sand interlocking. The post-liquefaction state of the soil specimen has been simulated using static loading after the dissipation of excess pore water pressure. It has been found that there was a sharp increase in the elastic modulus of reinforced specimens than the unreinforced specimens under a post-liquefied state. The presence of fibres facilitated an additional tensile strength to the fine sand specimen. Also, the specimen was found to be most stable at a fibre content of 0.6%. The analysis has been made after comparing the stress-paths of the specimens.