Pole-sitter Buchan (HMO Hyundai) made a smart start to take an early lead ahead of Aaron Cameron (Garry Rogers Motorsport Peugeot).
However it didn't take long for 2019 TCR Australia champion Brown to work his way into contention, the Melbourne Performance Centre Audi driver getting by Cameron before climbing up on the back of the leader.
Brown's charge was brought undone on Lap 6, though, when he was forced to limp back to the pits with an issue on the right front of his car.
It initially looked like a tyre failure, however once in the pits it became clear the issue was a broken shock from some early bump and grind.
That left Buchan in control of the race, his final margin over second-placed Cameron nine-tenths.
"It's so good to get one for the team," said Buchan. "Man I wish I could swear, they work so hard.
"I'm happy with that. I used my brain in that one, managed the gap and did what I needed to do."
GRM Peugeot pair Jordan Cox and Dylan O'Keeffe finished third and fourth ahead of Zac Soutar (Team Soutar Motorsport Honda) and Nathan Morcom (HMO Hyundai).
Luke King (MPC Audi) was seventh, Tony D'Alberto (Wall Racing Honda) eighth and Brad Shiels (Royal Purple Racing Hyundai) ninth.
The battle for 10th – and pole for tomorrow's partial reversed race – was an action-packed affair.
Initially it looked as if Kody Garland (GRM Renault) was in the box seat, until he got into the back of Fabian Coulthard (Wall Racing Honda) on Lap 11, dropping them both to the back of the field.
Michael Caruso (Ash Seward Motorsport Alfa) then took over 10th only for his right-front tyre to fail later on the same lap.
In the end it was Jay Hanson (MPC Audi) that scored the 10th and pole for tomorrow's second race, which kicks off at 11:20am local time.
