Burial diagenetic processes largely control sonic velocity and porosity destruction in the Oligocene-Holocene Seaspray Group carbonates of the Gippsland Basin. Extensive cementation of the carbonates begins at around 300-500 m burial depth, and most macroporosity is filled by calcite cement at a burial depth of around 1.5 km. Sonic velocity data from well logs are strongly correlated with the burial diagenetic processes operating in the carbonates. Above 300-500 m burial depth, sonic velocity increases rapidly with depth, with porosity loss being dominated by mechanical compaction. Below 500 m burial depth, the sonic velocity increases at a lesser rate, with porosity loss being dominated by pressure solution and calcite cementation. At these greater depths, sonic velocity (and therefore porosity) is a function of burial depth, carbonate content, and, to a lesser extent, sediment age. Lateral sonic velocity variations within the Seaspray Group are associated with submarine canyon facies. The submarine canyons are filled by sediments that have a higher carbonate content than the surrounding succession, During burial, these carbonate-rich canyon-fill sediments are more rapidly cemented by calcite, producing higher sonic velocities than the surrounding strata. The resultant velocity variations cause problems in time-depth conversion.