Macadamia flower blight complex is caused by several species of Botrytis, Cladosporium, Neopestalotiopsis, and Pestalotiopsis, resulting in severe yield loss to the industry. Botrytis cinerea is the causal agent of grey mould in various crops and has been extensively studied worldwide. However, there is limited knowledge about its pathogenesis in Australian native plants, such as macadamia. Therefore, this study investigated the factors contributing to the infection and pathogenesis of B. cinerea in floral tissues of the susceptible macadamia cultivar HAES 344. We examined the infection process from the initial point of contact to the symptom expression phase. Conidia germination, penetration, colonisation, and sporulation were observed from 3 hours post-inoculation (hpi) to 48 hpi using scanning electron microscopy (SEM) and light microscopy techniques. At 3 hpi following spray inoculation on the flowers, B. cinerea produced a single terminal germ tube, and the hyphae directly penetrated the petal epidermal cells without forming appressoria by 9 hpi. Extensive colonisation of the petal tissues by hyphae was observed at 24 hpi, along with crystal deposits adjacent to the infection sites. SEM and qualitative energy-dispersive X-ray spectroscopy analysis confirmed that the crystals were composed of calcium and oxygen, specifically calcium oxalate. This suggests that oxalic acid plays a role in the pathogenesis of B. cinerea in macadamia. A copious amount of conidia was produced from the infection site within 48 hpi, indicating a polycyclic epidemic behaviour in macadamia flowers. Therefore, preventing B. cinerea infection is critical to managing grey mould in macadamia.