A pre-stack simultaneous inversion in the Cooper-Eromanga Basin has produced very good predictions for all elastic properties: acoustic impedance (AI), shear impedance (SI) and density (Rho). The predicted relative elastic properties show very high correlations (>80%) with well data and surprisingly higher correlations for density than the other elastic properties. For many reasons, direct and accurate estimation of density from seismic AVO is expected to be difficult (e.g. Lines, 1998). Good quality predictions of density in this case are driven by constraints within the inversion. It is common to constrain density through a relationship with acoustic impedance such that when acoustic impedance is well predicted, density will also be reasonably predicted. Standard model based simultaneous inversions suffer from a lack of flexibility in setting constraints, which can be overcome through facies based simultaneous inversion where the relationships between the elastic properties vary with facies. When the facies can be accurately delineated from the seismic, very good predictions of density can be achieved even though the seismic data do not resolve the density directly. The higher correlations achieved for density than acoustic impedance are due to the larger dynamic range of density than for p-velocity.