AbstractThe East Australia Current dominates the circulation along the east coast of Australia; therefore, identifying observations that best constrain its transport and eddies may help improve circulation estimates. Observational data sets are sparse in time and space and numerical models are unable to predict the timing and location of eddies due to their chaotic nature. Data assimilation combines observations with a numerical model such that the model better represents the observations and provides the dynamic context. This study uses variational methods to quantify how oceanic observations from various platforms impact model estimates of transport and eddy kinetic energy in the East Australia Current. The most influential observations are, in this order, satellite‐derived sea surface temperature; radial components of sea surface velocity from an high‐frequency radar array midway along the coast; satellite‐derived sea surface height, temperature, salinity, and velocity observations from a full‐depth mooring array in the upstream portion of the domain; and subsurface hydrographic data measured by ocean gliders. Not only do the high‐frequency radar observations have high impact on transport estimates at the array location, but also they have significant impact both upstream and downstream. Likewise, the impact of the mooring array observations is far reaching, contributing to transport estimates hundreds of kilometers downstream. The observation impact of deep gliders deployed into eddies is particularly high. Significantly, we find that observations taken in regions with greater natural variability contribute most to constraining the model estimates.