The visiting side produced a near-flawless opening quarter to outscore the Mystics 15-10 and never surrendered the lead.
The 50-43 victory has lifted the Steel into third place, and a win against the unbeaten New South Wales Swifts in Sydney on Monday may be enough for the franchise to secure a home semifinal.
The seven-point triumph completed a run of four consecutive wins, the Steel's longest winning streak in the history of the tournament.
With each week, the Steel seems to lift its intensity a notch and execute the game plan with increasing accuracy.
And the belief is starting to swell, captain Megan Dehn said.
"We were really solid tonight and stuck to our structure really well," she said.
"There were moments where we wavered and got a bit hesitant.
"But we stuck it out and had really good connections on the floor and stepped up when it counted."
If the Steel was guilty of any wrong-doing it was sitting on its lead.
But the combined Otago-Southland team did more than enough in the opening 45 minutes to claim the competition points.
"For us it was good to have that five-goal lead because they were chasing for most of the game and we could just concentrate on our own game plan."
The Steel lost to the Mystics in round eight in Invercargill and was determined to make amends for a disappointing showing.
"Everyone was disappointed with themselves after that game. We took a look at ourselves and figured out what we need to do individually to pick up our game.
"We've really improved our effort on court since."
The teams traded three goals apiece before the Steel got an opportunity to open a lead when Cathrine Latu fumbled a pass under the net.
The Steel was more accurate in the first stanza, nursing possession and pouncing on anything loose.
The Mystics showed touches of brilliance but were then woeful shortly after.
If they were scratching their heads wondering why they were trailing by five at the break, they only needed to look at the turnover count.
The Steel turned the ball over a measly three times, while the Mystics made nine errors.
The home side lifted its intensity and made small inroads into the lead.
However, much of that good work was undone with Steel defenders Leana de Bruin and Te Huinga Reo Selby-Rickit doing a superb job slowing down Latu's supply of ball and picking off intercepts.
Mystics coach Te Aroha Keenan introduced experienced wing attack Jenny-May Coffin and defender Kayla Cullen at halftime but the changes did not have the impact she was looking for as the Steel stretched its lead to eight goals.
The under-rated Steel midcourt of Liana Barrett-Chase, Wendy Frew and Erika Burgess controlled the tempo and out-performed their highly-rated opponents.
Shooter Daneka Wipiiti had another fine game, landing 34 of her 39 attempts, but de Bruin was the star, clinging to the dangerous Latu and shutting down her space.
