context: The principles of imperialism are often generalizable to the policies and practices of the British Empire "during the last generation, and proceeds rather by diagnosis than by historical description". British imperialism often used the concept of Terra nullius (Latin expression which stems from Roman law meaning 'empty land'). The country of Australia serves as a case study in relation to British settlement and colonial rule of the continent in the eighteenth century, as it was premised on terra nullius, and its settlers considered it unused by its sparse Aboriginal inhabitants.
question:  What law did not justify British imperialism?