The info below was provided by Kevin Taylor of FootyStats:
In 1991, the AFL Commission introduced a procedure to prevent any final, other than a Grand Final, being tied. The procedure:
If a Grand Final is tied at the end of regular time, the game will be replayed.
In all other finals, if the game is tied at the end of regular time, extra time consisting of two five-minute halves plus time-on will be played.
Teams will maintain the same ends as at the completion of play for the first five-minute period of extra time then change ends.
If the score remain tied after extra time, additional period of extra time will be played until a result is achieved. Each additional period will also consist of two five-minute halves plus time-on.
Extra-time has been necessary only in one final since the rule was introduced -- it happened in 1994. Footystats Diary recorded it at the time --
QUALIFYING FINALS—September 10-11: Cloudless skies following mid-week rains greeted the first series of the Final Eight system, amid the continuing controversy of the Subiaco dustup. Only 38,223 were at the Park on Saturday afternoon to witness the 2nd QF in which the Kangas and the Hawks were the first to experience the extra-time rule. With scores tied at 91pts all at full-time, five minutes plus time-on was played to each end to break the deadlock. Earlier, Hawthorn with JOHN PLATTEN in his 200th as a shining light threatened to go away. The strength of WAYNE CAREY (NM) emerged to bring the Kangas back and an all-day challenging fight resulted. It was Carey with a 50-metre goal at 26m22s of the last quarter which tied the scores. In extra-time however, North surged constantly with eight-scoring shots. holding the Hawks pointless as the Kangas ran in for a 23pt victory and their first finals win since 1985, NORTH MELBOURNE 15.24-114, HAWTHORN 13.13-91
Footystats Daily Diary