The changes in seawater volumes caused by seawater intrusion are often neglected in coastal aquifer management studies. The conditions under which this can result in significant water balance errors are not well understood. Interface movements also influence temporal trends in coastal aquifer water levels, but there is little guidance on this effect. In this study, we use steady-state, sharp-interface, analytic modelling to generate idealised relationships between seawater volume, freshwater volume and water levels. The approach assumes quasi-equilibrium conditions, which are evaluated using a selection of transient, dispersive simulations. The results demonstrate that seawater volume changes can influence significantly coastal aquifer water level trends, relative to the corresponding non-coastal aquifer situation, particularly within deep aquifers with high hydraulic conductivity and low net recharge. It is also shown that seawater volume changes can be a significant component of coastal aquifer water balances, e.g., relative to freshwater discharge to the sea, especially within deep aquifers characterised by low hydraulic conductivity and low freshwater discharge. Transient simulations show that steady-state conditions are a reasonable approximation for a range of transient seawater intrusion situations, including two of the three cases considered in this analysis. We conclude that changes in seawater volumes should be included routinely in coastal aquifer water balances. Also, temporal trends in coastal aquifer water levels may not provide an adequate measure of freshwater storage trends. It follows that the assessment of coastal aquifer condition should consider groundwater levels relative to the hydraulic head imposed by the ocean, accounting for density effects.