The seasonal pattern of adult fly production in both fresh and aged biosolid cake was established by the exposure of biosolid cake to natural conditions in the central coastal region of Perth, Western Australia, over the course of a year. Relationships between fly emergence and six factors (weeks exposed, pH, NH4, rainfall, temperature, and moisture) suspected of facilitating the attraction of adult flies to biosolid cake were measured. The aim was to study the production of adult flies with regard to weeks exposed in different seasons to establish a “best practice” storage procedure for aged biosolid cake. Four fly species were collected during the study; Musca domestica L. (house fly), Stomoxys calcitrans L. (stable fly), Fannia canicularis L. (lesser house fly), and Muscina stabulans (Fallén) (false stable fly). Of the 5,303 flies identified, >97% of these were either M. domestica or S. calcitrans. Adult flies emerged from biosolid cake all year round with a sharp decline in winter. The number of emerged flies peaked in summer (3,163 flies) and declined during autumn (1,545 flies) and spring (532 flies). Significant positive relationships were observed between the number of adult flies emerging from biosolid cake and ambient temperature, NH4 content, and weeks exposed. Higher ambient temperatures and a higher NH4 content of the biosolid cake were associated with increased number of adult flies emerging from the biosolid cake. Similarly, the majority of flies emerged from biosolid cake in the first 10 wk of exposure to fly activity.