The volumetric relationship between the intrusive and extrusive components of volcanic fields is difficult to constrain. This is because either the erupted products are commonly eroded or the intrusions are concealed by overburden. Three-dimensional seismic data therefore offer a unique opportunity to study ancient volcanic provinces, as these datasets can image both components without the need for erosional dissection. This study is based on 3D seismic data from the Bight Basin Igneous Complex (BBIC), a volcanic province constructed within a sedimentary basin, offshore southern Australia. We demonstrate that the BBIC contains abundant sills and laccoliths emplaced within deltaic sediments. The intrusions have a total volume of 92–111 km3, and the lava flows and volcanoes have a combined volume of 66–76 km3 (Dense Rock Equivalent). This indicates that the ratio of intrusive:extrusive products for the BBIC is between 1:1 and 2:1. The architecture of the BBIC contrasts markedly with the neighbouring Newer Volcanics Province (NVP), which lacks abundant sills and laccoliths. Because lava flows and volcanoes in the NVP were dominantly fed by dykes hosted in basement rock, we suggest that subsurface lithology plays an important role in causing sill formation and determining the total volume of magma stored in the subsurface.