The goal-scoring slowed in the second half as both sides had chances but their scrambling defence held strong until the final minute of the match when Cooper got her fifth.
The battle between the third and fourth-placed teams had little riding on it in terms of how they would finish the season on the table but since they are likely to face each other in the first round of playoffs, Forrester believed the win will give Maungakaramea the edge in coming weeks.
In other premier women's games, Springfield showed they mean business leading into playoffs by beating Mangapai 12-0 while Old Girls beat Hikurangi 5-1.
In the premier men's competition, Mangapai beat Whangarei Boys' High School 3-1 and Maungakaramea beat Bream Bay 5-2.