This study assessed the size-related patterns of dietary resource use in terapontid assemblages from two north Australian wet-dry tropical river systems exhibiting contrasting long-term flow regimes. Substantially higher size-related dietary divergence in the form of additional trophic guilds and lower levels of dietary overlap were evident among terapontids from the comparatively stable Daly River system, particularly during the dry season. Relatively restricted expression of size-related dietary shifts in conjunction with higher levels of dietary overlap and no significant seasonal effects on dietary overlap were features of assemblage interaction in the highly variable Burdekin River. Results highlight size-related dietary shifts as a complex aspect of terapontid trophic habits, and one that can exhibit considerable spatial dynamism.