Timely outbreak notification is critical for successful disease control. For high pathogenicity avian influenza (HPAI) and African swine fever (ASF), surveillance performance within and across countries remains largely unknown, despite their continued global spread. We assessed surveillance performance in reporting HPAI outbreaks (2020-2023) and ASF outbreaks (2016-2023) amongst World Organisation for Animal Health (WOAH) member countries/territories using WOAH outbreak notification data. We employed a modelling approach using the number of fatalities reported at initial notification as a performance measure, where fewer fatalities relative to the size of the affected premise-after accounting for other premise-level and country-level factors-were assumed to indicate better performance. For both diseases, fatalities were strongly associated with factors including country/territory, number of susceptible animals, premise type (commercial farms, backyard farms, and villages), season, and spatiotemporal outbreak clustering. The number of susceptible animals and country/territory explained the most variance in fatalities. While a few countries/territories appeared to perform exceptionally well or poorly, significant overdispersion suggested substantial heterogeneities within countries/territories after controlling for other factors. Our findings highlight the need for more targeted national and global efforts to strengthen animal health surveillance capacities.