The Roar secured top spot with three games of the normal season to spare and did so in style, a hobbling Brattan rifling home from distance deep into stoppage time to see off the second-placed visitors.
It is the second time that the club have won the plate and they will be hot favourites to prevail in the final series.
The hosts created the better chances throughout but Victory stubbornly held on until the death, when Brattan, struggling with a hip injury, wrapped up three decisive points.
Henrique Silva was the first to test Lawrence Thomas but the Brazilian's swerving shot was parried before the Victory goalkeeper blocked Dimitri Patratos' follow-up effort.
Liam Miller spurned a glorious chance to open the scoring inside the opening 15 minutes as he fired straight at Thomas.
A similarly concerted rearguard effort kept the home side at bay as Henrique and Jack Hingert were both denied.
But Brattan proved to be the hero when he powered home a superb shot from 25 yards with just seconds to spare.
Newcastle Jets secured much more emphatic triumph as they thrashed Wellington Phoenix 5-0.
Former England striker Emile Heskey opened his account for the club, his first goal in more than a year, with strikes from Joel Griffiths, Michael Bridges and Adam Taggart's double sealing the result.
