Simeon has wide experience of international relations, defence and intelligence issues, serving in senior positions at the Australian Signals Directorate (ASD) and the Departments of Defence, Foreign Affairs and Trade (DFAT) and Prime Minister and Cabinet (PM&C). 

Since retiring from the Australian Public Service in 2019, Simeon divides his time between farming, consulting and his work as an ASPI senior fellow, where he has written on national security-related technology issues.

Prior to leaving Canberra, Simeon was Deputy Director-General for Signals Intelligence and Network Operations in ASD. In 2014-15, he was Senior Adviser (Defence and Security) in Prime Minister Tony Abbott’s office following a brief stint as chief of staff to the Minister for Defence.

Between 2008 and 2013, Simeon held several leadership posts in the Department of Defence, serving as Acting Deputy Secretary Strategy (Operations), where he led policy support to military operations in Afghanistan, and later as Deputy Director for Intelligence at the then-Defence Signals Directorate. Prior to that, Simeon headed Defence’s International Policy Division.

Before joining Defence, Simeon spent much of his career at DFAT with diplomatic postings to Thailand and Myanmar. Between 2004-07, Simeon ran the Consular Branch where he provided assistance to Australians in difficulty overseas. From 2002 to 2004, Simeon led the Asia, Americas and Trade Branch in the International Division at PM&C.