Recycled plastics have poor mechanical properties but when blended with fillers, these properties can be significantly improved. This study involves the addition of fillers, including mixtures of talc, calcium carbonate and fibreglass, to improve the mechanical properties of recycled high density polyethylene (HDPE). The filler percentage is kept constant at 30 wt%. The plastics and fillers are initially mixed together using a twin screw extruder and then granulated using a pelletiser. The granules are fed into an injection moulder, producing standard sized test samples. The mechanical properties of the polymers increase with higher fibreglass content. For a secondary filler together with fibreglass, talc is superior to calcium carbonate providing a distinct improvement in mechanical properties, when comparing the two fillers. Aside from the 30% 3mm fibreglass, the best filler combination found is 20% 3mm fibreglass with 10% Talc T45B, improving the flexural strength, flexural modulus, tensile strength and tensile modulus by 80.2%, 187.2%, 45.1% and 193.7% respectively over the base HDPE. The results generally followed the expected theory, however a few anomalies were found, including the peaking of certain properties at different concentrations when using various filler combinations, and the differences in results between the 3mm and 6mm fibreglass. Some of these anomalies are understood to be caused by the ineffective dispersion and orientation of fibreglass inside the polymer matrix.