Which team was relegated from the league at the conclusion of the 1991 Season?

Context: Waterside Karori were relegated at the end of the 1990 season, to be replaced by the winner of a play-off series between teams from the northern, central, and southern leagues. Nelson United won the series to gain promotion, but there is some controversy about their participation in the play-offs as they had only finished second in the central regional league, which was won by Petone. 

Gisborne City were relegated at the end of the 1991 season

Answer: Gisborne City were relegated at the end of the 1991 season.