This article argues that, rather than viewing Melanesia as an “arc of instability”, Australia should see it as an “arc of responsibility”. The idea of responsibility makes explicit the central role that Australia seeks in the region, and the idea of the arc reflects Australian perspectives. By its actions, Australia has extended its formal security guarantee to Papua New Guinea to the rest of Melanesia and Timor-Leste. The nature of the guarantee has expanded beyond protecting external security to a range of commitments to maintain the internal stability of these states. In an ad hoc manner, Australia has expanded its role as security guarantor to match its position as the region’s largest aid donor. Yet Australia still has far to go before its understanding of its economic role in the arc matches its security guarantees. And Australia’s leadership ambition does not always get much regional “followship”.