In September 2011, Müller was appointed Ambassador to Australia and succeeded Michael Witter, who was made ambassador to Morocco. As ambassador to Australia, he received non-resident accreditation for Papua New Guinea, Vanuatu, Nauru and the Solomon Islands. As ambassador during the First World War centenary, Müller participated in commemorations for the 1914 Battle of Cocos and stated that "In both countries we reflect on the failure of politics and diplomacy in 1914, and on the conclusions to be drawn to jointly prevent any recurrence of such a catastrophe in today’s world." In April 2015 Müller became a member of the newly established Australia-Germany Advisory Group which "will work to identify opportunities for increased trade and investment, cooperation in the Indo-Pacific and other regions of the world, cultural and social links and scientific collaboration." He was active in promoting ThyssenKrupp Marine Systems' bid for the Royal Australian Navy's Collins-class submarine replacement project.