South Melbourne were the defending champions, having defeated Sydney Olympic to win their first title in the previous year's final, but they were eliminated in the semi-finals by eventual winners Melita Eagles.
Melita Eagles defeated Preston Makedonia 1–0 in the final to win their first NSL Cup title.
The NSL Cup was a knockout competition with 14 teams taking part all trying to reach the Final in April 1991.
[1] The competition consisted of the 14 teams from the National Soccer League.
Melita Eagles and Preston Makedonia had a bye for the First round.