Palaeomagnetic data obtained from studies of rock formations in southeastern Australia are compared with data from the main Australian assemblage. Occurrences and distributions of some of the major serpentine and ultramafic belts in the Tasman Orogenic Zone are interpreted according to current plate theory as marking sites of ancient plate margins. A tectonic model based on the palaeomagnetic data with the aid of geological constraints is proposed. It describes the rotation of a small region in southeastern Australia, relative to the main assemblage, by about 90° since the Middle Silurian and 30° since the Early Devonian. The proposed small plate boundaries may be represented by the Great Serpentine Belt to the east and in part by the less well defined belt of serpentinites which outcrop from Kiandra to Nyngan on the western margin. It seems unlikely that the whole of the western serpentine belt was actively involved with accretion during the Siluro‐Devonian. The northern limit of the plate cannot yet be denned. The significance of other serpentine belts may be assessed by extending palaeomagnetic investigation to include those regions suspected of having an active plate history.