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Wiktionary
boundary rider

alt. 1 (context Australia English) A person employed to travel along the boundary of a station, to check and maintain the fencing. 2 (context Australia sports Australian rules football English) A commentator on Australian rules football games who reports from the boundary of the field of play, and interviews players on the interchange bench as well as coaching and medical staff. n. 1 (context Australia English) A person employed to travel along the boundary of a station, to check and maintain the fencing. 2 (context Australia sports Australian rules football English) A commentator on Australian rules football games who reports from the boundary of the field of play, and interviews players on the interchange bench as well as coaching and medical staff.

Wikipedia
Boundary rider

A boundary rider is a term used in the Australian Football League as well as other field sports to denote a commentator who works from the sidelines of the field or 'boundary'. It is a recent extension of the far earlier, Australasian use of the term "boundary rider"; i.e., one who rides round the fences of a station (hence "boundary"), checks their status, and repairs them when broken.

The role of the boundary rider is to have access to the players and coaching/medical staff on the interchange bench enabling them to provide more detailed commentary on current game plan as well as any injury concerns for the competing teams. They often also have the responsibility of interviewing players and coaching/medical staff during breaks in the games (quarter/half/three quarter time) as well as post conclusion of the match.