Buchan made a swift start from pole in his sedan-spec Hyundai, immediately gapping the chasing field led by Honda driver D'Alberto.
Behind him there was early drama, Brown, running fifth in his Melbourne Performance Centre Audi, slowing on the main straight on lap 2 with a broken right rear upright.
The race then settled into a rhythm at the front, with Buchan comfortably leading D'Alberto and Audi driver Zac Soutar.
That was until lap 8 when Soutar's car began to misfire which forced him into the pits and out of the race.
That promoted a hard-charging Lachlan Mineef (Forza Brakes Motorsport Audi), who had worked his way up from the back of the top 10, to third place.
Mineef wasn't done there, though, the privateer then hunting down D'Alberto before squeezing past at MG to grab second.
Moments later D'Alberto's race was run, the reigning champion suffering a front-right tyre failure on the way into turn 1.
None of the later drama affected race-long leader Buchan, though, who cruised to victory with a 2.1s margin over Mineef.
"That was amazing," said Buchan. "I didn't expect that coming into the weekend.
"The damage was done early on on that first lap, I got the greatest launch of my life. The tyres went off at the end there. There's more to come, but I'm just happy to get a win."
Aaron Cameron (Garry Rogers Motorsport Peugeot) benefitted from D'Alberto's bad luck, ending up third.
Series leader Bailey Sweeny (HMO Customer Racing Hyundai) ended up fifth ahead of Ben Bargwanna (GRM Peugeot), Michael Clemente (Carl Cox Motorsport Cupra), Kody Garland and Jordan Cox (both GRM Peugeot).
Newcomer Brad Harris finished ninth in his Wall Honda, while Tom Oliphant was classified 10th despite his Lynk and Co coming to a stop two laps from home.
D'Alberto was a DNF, despite trying to get back out on a new tyre right at the end, while Brown was left to lament his extinguished title hopes after a fourth DNF from the four races so far this season.
"If I don't laugh, I'll cry," said Brown. "This is pretty much a new part and the control arm has torn out of the upright. I don't know how it happened, I didn't hit anything and it was a pretty clean start.
"Overall it's not a good weekend, I haven't finished a single session. Four DNFs, you're not going to win the championship."
The weekend continues with race 2 at 11:30am local time tomorrow.
