Gambier-Portland Basin has developed by step faulting. It is separated from Port Fairy - Otway Embayment by postulated Grampian Fault. Tertiary may attain a thickness of 10,000 in Portland - Cape Bridge area. Tilted and flexed block faults are main structural feature; major fold developments are probably absent. Structural possibilities for oil are thought to be restricted to deeper parts of the basin where differential movement of fault blocks may produce traps in Jurassic. Mt Gambier may be badly faulted with volcanic intrusion complications. Its consideration as a potential oil structure is not warranted at this stage.