The Roar took a 2-0 lead into the return leg at home and again overcame the Mariners with a 2-1 victory (4-1 on aggregate) and their fourth win on the trot.
Young striker Mitch Nichols again opened the scoring with an opportunist effort from a Massimo Murdocca drive in the 22nd minute.
Nichols then became the provider when he played Matt McKay through who finished coolly to put further doubt into the mind of the Mariners with a goal four minutes after the break.
The Mariners tried in vain to get forward and it finally paid off in the 64th minute when Sasho Petrovski played the ball to Nik Mrdja just outside the area and on his trusty left foot crashed one past Liam Reddy, using the greasy surface to his advantage.
The visitors peppered the goals from then on but a resilient Roar outfit managed to hold them out and qualify for a chance to make the Grand Final.