Species assemblages can be used as surrogates to monitor impacts on the distributions of multiple species from environmental changes and/or anthropogenic activities. In this study, we recorded seabirds at-sea in the northern and southern extremes of mainland Aotearoa New Zealand. During 2019–2023, transects off Northland were undertaken in summer (n=4 trips), winter (n=2) and spring (n=3), recording 23 species and 2,839 birds in total. Off Otago, a 60-km transect was surveyed bi-monthly, during 2015–2023, in summer (n=14), autumn (n=7), winter (n=10), and spring (n=5), recording 48 species and 69,025 birds. We specified model-based ordinations to understand (dis)similarity among seabird observations and assemblages. For both datasets, environmental characteristics of the transects did not explain assemblages as well as ‘season’ alone. Most of the species recorded in both datasets are migratory. The seasonal patterns become apparent when comparing relative abundances and frequencies of occurrence. In a changing ocean with emerging anthropogenic threats such as offshore renewables, effectively identifying and monitoring species assemblages is crucial for management. Our study highlights the need to account for seasonal changes in seabird occurrence when designing a monitoring framework.