For example, this conference (https://www.monash.edu/tenor2019/committees) is being held in Melbourne, Australia; but the Program Committee consists of over 40 members from interstate and overseas. Would it be expected for all of these committee members to be present at the conference, or could their roles be carried out without actually attending?
Program committee members are not expected to be present at conferences. They are responsible for reviewing submissions (before the conference) and helping to select submissions to be presented at the conference. Nonetheless, program committee members may attend, possibly even to present a submission that was selected (by other members of the program committee). Beyond the program committee, other committee members may be present, indeed, some must be present.