Sam Weston is an agent with the ASIS.
He holds Australian citizenship through his father, Andy, a former member of one of that nation's Special Forces groups and a man trained in warfare and survival. He also hold dual citizenry with the United States from his mother, herself an American Special Forces soldier for several years, and because he was born and partially raised in Hawaii. The adventure which brought his parents together is told in the prequel recounting which also describes in some detail how Weston would come to have as negative an attitude towards the CIA as he does.
Weston, as mentioned, was raised in Hawaii but he spent several summers with his father in the Outback. When he was 14 his mother was assassinated and he went to live permanently with his father which is how this American-born young man came to be a soldier a few years later in the Australia's armed forces and subsequently a valued member of that country's primary covert intelligence organization.
Weston is an interesting contradiction in that he really took to the huge expanse of the Outback and his father's very large and successful sheep and cattle station in the far western part of New South Wales while at the same time he seemed to thrive in various cities around the globe where he is sent on missions by the ASIS.
In addition to those assignments, he has a side business going on his down time, one which he refers to as his "little black jobs" for which he charges $10k a day plus expenses. These prove useful in his ASIS work as they "helped him maintain his networks in the shadier side of business" and they in turn were helped by the fact that working for ASIS have him access to criminal intelligence information. Some of these side jobs might be a tad sketchy but "he had a strong conscience and never took on anything grossly illegal". Toss in the fact that as an ASIS operative he could travel under a diplomatic passport and he was able to operate just about anywhere.