As outlined in the General Match Requirements, and varying according to the Age group, each team is required to provide at least one Goal Umpire for each game. This person is required to act according to the rules of the game (as outlined below) and talored for the age group and related game requirements.
The following information has been taken from the AFLs "Laws of Australian Football 2012"
8. Umpires: Duties and Instructions
8.1 Appointment and Duties of Umpires
Unless Law 8.1.2 applies, a Controlling Body shall appoint for each Match:
(a) a field Umpire or Umpires;
(b) two or more boundary Umpires; and
(c) two goal Umpires.
8.1.2 No Appointment of Boundary/Goal Umpires
(a) Where it is impractical to appoint or the Controlling Body has not appointed a boundary or goal Umpire to officiate at a Match, the field Umpire(s) may, in consultation with each Team, appoint persons before the commencement of the Match to officiate in those roles.
(b) Unless otherwise determined by a Controlling Body, a field Umpire may overrule the decision of a boundary or goal Umpire who was appointed under Law 8.1.2 (a).
(c) When no boundary or goal Umpire has been appointed under Laws 8.1.1 or 8.1.2 (a), the duties normally undertaken by a boundary and goal Umpire shall be undertaken by the field Umpire.
8.1.3 Replacement of Field Umpire
A Controlling Body shall adopt rules which prescribe the replacement of an Umpire who, by reason of injury or otherwise, becomes incapable of officiating either before orduring a Match.
8.1.4 Non-attendance of Field Umpire
Where a Controlling Body has not appointed a field Umpire or for any reason a field Umpire is unavailable or does not attend to officiate at a Match, the competing Teams may elect to play the Match and upon doing so, may appoint an Umpire(s) (inclusive of field, boundary and goal Umpires), to officiate at the Match. The relevant Controlling Body may determine in its absolute discretion whether to recognise the result of such Match.
8.2 Duties of Umpires
8.2.1 Field Umpires
The field Umpires shall officiate and have full control of a Match.
8.2.4 Goal Umpire
Unless otherwise determined by the relevant Controlling Body, the duties of a goal
(i) judging whether a Goal or Behind has been scored;
(ii) signaling that a Goal or Behind has been scored upon being given the All Clear or Touched
All Clear by a field Umpire;
(iii) recording the Goals and Behinds scored by each Team during a Match;
(iv) reporting a Player or Official who commits a Reportable Offence; and
(v) Assisting boundary Umpires to determine if the football is Out of Bounds.
(b) Behind or Out of Bounds
Where a goal Umpire considers that a Behind has been scored and a boundary Umpire
considers the football has gone Out of Bounds or Out of Bounds on the Full, the decision of the
goal Umpire shall prevail.
(c) Comparing Score
At the end of each quarter and at the end of a Match, the goal Umpires shall compare the
score which they have recorded. If the scores are different and cannot be agreed upon or
corrected by the goal Umpires, the scores shall be compared to the score recorded by the
Timekeeper(s) or person(s) appointed by the Controlling Body under Law 12.7 (if any). If the
matter still remains unresolved or the Timekeeper has not recorded the scores, the matter
shall be referred to the relevant Controlling Body for determination.
(d) Goal Umpire Unsure
If a goal Umpire is unsure whether the football crossed the Goal or Behind Line, or is Out of
Bounds; the goal Umpire shall seek the assistance of the Field and boundary Umpires. If the
correct decision cannot be determined following consultation, the goal Umpire shall give the
Reference Source: Laws of Australian Football 2012