This study investigates the mechanical behaviour of cemented rock fill and tyre-enhanced cemented rock fill, with the focus on how the particle size distribution influences compressive and tensile strength. Large-scale unconfined compressive strength and direct tensile tests were conducted using natural waste rock sourced from a mining operation. The cemented rock fill samples were modified by incorporating varying contents of shredded waste tyre rubber. Results indicate that increasing rubber content causes decreased compressive strength but enhanced tensile strength. When compared to existing literature, the results from the experiments highlight the influence of particle size distribution on the effectiveness of rubber inclusion in cemented rock fill in mines, particularly the finer fraction. It was observed that a minimum waste rock particle size, d45 = 20–25 mm is required for consistent improvement in tensile strength. This study offers valuable insights for developing sustainable mining backfills and for optimizing mix designs for optimal mechanical performance.