The magnetizations of many rock units in southeastern Australia have been severely overprinted. Interpretation of early work was hindered by this, and one of the aims of the current research is ascertain the nature of the overprint mechanism, its geological significance, and its regional extent. Results from three igneous bodies from Sydney Basin, which have been studied previously in a reconnaissance fashion, have provided a new insight into the reason for such widespread magnetic overprinting. The Hornsby Breccia (HB) and Milton Monzonite (MM) yield similar magnetic signatures, both containing discrete high- and low-temperature magnetizations with directions of dec = 72¿, inc = +83¿ (α95 = 4¿) and dec = 13¿, inc = -76¿ (α95 = 2¿), respectively, for HB, and dec = 60¿, inc = +78¿ (α95 = 4¿) and dec = 348¿, inc = -79¿ (α95 = 6¿) respectively for MM. The corresponding pole positions are 29¿S, 166¿E(A95 = 7¿) and 59¿S, 139¿E (A95 = 4¿) for HB, and 22¿S, 171¿E (A95 = 7¿) and 55¿S, 158¿E (A95 = 11¿) for MM. The paleomagnetic direction and pole from the third body studied, the Mogo Hill Basalt (MH), for dec = 241¿, inc = 81¿ (α95 = 5¿) and 41¿S, 130¿E (A95 = 9¿). This pole position and the two low-temperature overprint pole positions from HB and MM are consistent with ages ranging from 100 m.y. to approximately 70 m.y. when compared to the Late Mesozoic apparent polar wander path. We relate the overprinting to uplift, erosion, and rapid supracrustal cooling of the southeastern margin of Australia associated with initial phases of marginal rift development prior to seafloor spreading in the Tasman Sea. The temperatures of rocks now exposed may have reached 200¿C or higher, before the rapid cooling blocked and stabilized the overprint magnetizations.