Much research is being devoted to green bio-composites production and properties. The most popular materials in this field are wood plastic composites (WPC) based on polyolefins and different wood fibres. Our studies are focussed on investigating the properties (physical, mechanical and water uptake) of high-density polyethylene-based bio-composites, containing birch wood veneer byproduct from plywood production: and plywood sanding dust (PSD). The fluidity of composite melts was noted by melt flow index (MFI) measurements. Fracture mechanisms of the composites were investigated with scanning electron microscopy studies. For WPC modification, the coupling agent maleated polyethylene (MAPE) was used. WPC loaded with PSD showed improvement in tensile, flexural and micro hardness properties, but MFI, impact strength and deformation ability decreased with an increase in the filler content. By contrast, water resistance significantly increases with the presence of MAPE in the composites.