For decades, Australia’s naval acquisitions leaned heavily on British or American designs, with occasional forays into European shipbuilding. That initiative collapsed by 2016 amid concerns over Japan’s lack of experience exporting defence technology, capability mismatches, and political resistance within Australia’s defence establishment. The convergence of these factors creates a narrow but significant opportunity: Australia and Japan can move beyond ad hoc defence cooperation and formalise a defence alliance anchored in joint capability development. A joint submarine program would allow Japan not only to deepen its defence industrial base but also to institutionalise a new security partnership with Canberra. Diversify Australia’s defence dependencies , reducing overreliance on any single partner.