It was just the second win this season for the Cardinals, who went into the weekend with a 1-9 record at the foot of the premier men's competition.
Forrester took a ball, sliced two fouls and then took a second ball before putting veteran Ellis Park pitcher Pete Mathieson over the fence in the first at-bat of the game.
Simon Maole added a two-run homer in the first inning to give the Cardinals immediate control against Ellis Park.
Whaanga's big blast came at the top of the third, when he saw off Mathieson with a 1-1 count.
Cardinals led 6-1 going into the bottom of the fifth inning, with young pitcher Joshua Ludlow doing a sterling job on the mound.
Ludlow coughed up two runs before each side got a run in the sixth, making it 7-4 with one to play.
The Cards could smell blood by now and added two more runs at the top of the seventh, with a Doug Hill double scoring Dave Arthur, and a Forrester double bringing home Hill.
There were no such heroics for the Cardinals in the early game against the Saints, which ended in a 13-1 win to the second-placed side.
The Saints led 2-1 after two and a-half innings but smashed the game open with five runs at the bottom of the third and six at the bottom of the fourth.
Mike O'Neill provided the moment of the day when he smashed a grand slam (bases loaded) homer off Ludlow in the third inning.
The Saints backed up with an upset of their own, 5-3, against the competition-leading Dodgers, who had lost just one game all season.
Sam Jones pitched six clean innings for the Saints, his only blemish a three-run fifth inning when he walked four batters.
With the bat, the Saints were effective rather than spectacular, scoring one run in each of the first four innings and in the sixth.
The Dodgers produced one big inning to beat Ellis Park 6-2 in the other game of the doubleheader round.
Both teams' bats were cold through the first three innings before the Dodgers leaped into the lead in a chaotic four-run, four-error fourth inning.
Ellis Park responded with a two-run Chang homer but the Dodgers pushed further ahead with runs in the fifth and six innings.
