Petrophysical and petrological studies of the Mogo Hill diatreme in the northern Sydney Basin show significant features in alkali basalt and volcanic breccia. Up to 5% multidomain titanomagnetite occurs in the basalt with Curie Temp. 250 °C. The basalt has an average magnetic volume susceptibility of 3405 × 10−6 cgs and an average density of 2.97 gms cm−3 which contrast with the associated volcanic breccia’s average susceptibility and density of 510 × 10−6 and 2.43 respectively. The sandstone country rock has negligible susceptibility and density of 2.29. The sandstone average total porosity, 14.5%, is much greater than the volcanic rocks which average 2.8%. The basalt has strong remanence with Qn av. 1.59 giving a reversed anomaly of 800 gammas over the deposit which appears dyke like on the evidence of magnetic trends and anisotropy data. It is considered that complete magnetic and gravity studies should help resolve the detailed geometry of dense and magnetic intrusions such as Mogo. Petrological and geochemical evidence from the alkali basalt, the breccia and the abundant xenoliths, indicates that expansion of magmatic CO2 and H2O rather than a phreatic mechanism accounts for the fragmental character of the diatreme. The explosion was probably initiated at about 5 km depth. Mantle and crustal zenoliths entrained in both the basalt and breccia aid interpretation of the nature of the upper mantle and crust beneath Mogo.