Wastewater treatment plants (WWTPs) are an important pathway of microplastics to aquatic and terrestrial environments. Application of treated sludge (biosolids) for agricultural use has been suggested as a potential source of microplastics to cropping lands. In Australia, agriculture is the largest biosolid end-user due to the many benefits biosolids adds to the soil, however, there is limited understanding of microplastic abundance in Australian biosolids. In this study, we analysed microplastics in biosolids from five WWTPs in Queensland over two seasons. We have developed and optimised an oxidative-enzymatic purification method to overcome current limitations of organic matter removal from biosolid and accurately analyse and report microplastics in biosolid with minimum bias. This method enabled removal of >95% of dry weight of solid material, without loss of microplastics. The concentration of microplastics (>20 µm) in all samples ranged from 11 to 105 and 23 to 62 particles/g dry weight over cold and warm seasons, respectively. A total of 15 different polymers were identified using Laser Direct Infrared (LDIR) spectroscopy with polyethylene terephthalate (PET), polyethylene (PE), polymethyl methacrylate (PMMA) and polyurethane (PU) comprising >60% of total polymers. Smaller microplastics between 20 to 100 µm comprised >80% of microplastics across all samples. Microplastics were mainly found in the form of fibres (average 56%) and fragments (average 53%), with a higher abundance of fibres detected during the cold season. Given the annual production of biosolids (12000-55000 t/year), between 3.6×1010 to 4.7×1012 microplastic particles enter the terrestrial environment from biosolids application each year. Therefore, biosolids are an important source of microplastics to the environment, emphasising the need to more comprehensively understand the fate, impact and risks associated with microplastics on agricultural soils.