Seen as imposters for a time, they have proved their credentials with wins over every team in the competition - except the Mystics. The Auckland side won that dramatic round eight clash 49-45 - the match mostly memorable for the 'Harrison Hoist'.
"We learnt a lot that night," says McMahon, "both about ourselves and the Mystics.
She admits the Mystics have "incredible strength" across the roster but is confident they have the game plan to topple the 2011 grand finalists.
"It is tough to assess what they will do," says McMahon. "You can plan for one combination and then they will spring another one on you, as they have such depth.
"We will be concentrating on defending across the full court and it is imperative we defend as a team.
"Our defence this season has been a real feature for us and we expect that to continue.
"On attack, the keys are quick ball movement, speed on release and to always keep the ball moving. Those are the things that will win the game for us."
If the Vixens have an Achilles heel, it is at their shooting end. Even at this stage of the season, they are still not certain who their best duo is. Teagan Caldwell, Kate Beveridge and Karyn Howarth have all had sweet and sour moments without a consistent shine.
"It hasn't been as settled as we wanted it to be," says McMahon. "But I am confident that they have hit their straps now. In the middle part of the season, we were struggling with out starts, though in the last two matches, we have exploded in the first quarter."
Caldwell sometimes resembles a McMahon Mini-Me and it is easy to see the influence of the former Australian great in the way she aggressively comes to receive the ball.
McMahon, who is due to give birth at the end of August, is keeping an open mind about what her playing future may hold in 2013.
She would have a short window before pre-season training but it wouldn't pay to bet against the 34-year-old resuming her career.
"I've been told that having a baby changes your life," laughs McMahon, "so I am keeping an open mind on what I may do.
"Certainly, though, the way it all ended [with injury] does not sit well with me, so there is definitely a part of me that wants to write another chapter."
For her part, Mystics coach Debbie Fuller is looking forward to another engrossing battle in Melbourne.
"Victorians are extremely staunch supporters," says Fuller. "But they also appreciate good netball, so it is a pleasure to play there. We are obviously not underestimating the Vixens but we are looking forward to playing them."
There is a sense the Mystics have a much greater stomach for the fight on Australian soil, especially after breaking their transtasman hoodoo last year.
"I think we are comfortable now with our strategies against Australian teams," says Fuller. "We know what to expect."
The equation, on paper anyway, is not a complicated one. If the Mystics put together four quarters of quality netball and play to their undoubted potential, a second grand final is just 60 minutes away.
If they don't, they'll have the nervy prospect of a red-hot Magic team or a resurgent Thunderbirds side next week.
"People get desperate in finals situations," says Fuller. "It might all come down to one goal, one play, one moment and who wants it more."
