Mostert was into a clear lead before the field arrived at Turn 2, with Waters failing to lead into the opening turn for the first time this weekend.
The #25 Mobil 1 Appliances Online Commodore was the quicker of the front-running duo, with the #6 Monster Energy Mustang keeping a watching brief.
Unlike the day’s earlier Race 7, the field managed to navigate the opening lap cleanly, with the Shell V-Power Racing Team duo of Anton De Pasquale and Will Davison both eyeing the final spot on the podium.
Zane Goddard, who launched from a career-best fourth, managed to keep the all-conquering Red Bull Ampol Racing entries behind, with Shane van Gisbergen unseating Jamie Whincup for sixth on lap 5.
WATCH: BULLS GO INTO BATTLE
Van Gisbergen was the first of the leading pack to make his compulsory pit stop on lap 8, with Davison and Whincup stopping on the following lap.
De Pasquale followed Waters into the lane on lap 10, with Mostert staying out in front.
As soon as Mostert began to hit traffic, WAU pulled the pin and called the race leader in on lap 13.
He emerged with a three-second gap over Waters, but quickly began to build a gap as the Monster Ford fell into the clutches of De Pasquale.
Van Gisbergen and Whincup both jumped Goddard in the pit cycle, with seven seconds covering the top seven as the field cleansed.
WATCH: REYNOLDS’ CLOSE CALL
By lap 31, Mostert led Waters by seven seconds, with De Pasquale and Davison ranging up behind.
By the third-to-last lap, Waters, De Pasquale, Davison, van Gisbergen and Whincup were covered by just 1.4s.
Davison made the move stick at Turn 6 ahead of the penultimate lap, with De Pasquale hurrying Waters along.
Van Gisbergen went shallow on De Pasquale at the hairpin, with the #11 Shell Ford repeating his teammate’s manoeuvre to shift Waters off the podium.
The championship leader tried to unsettle Waters on the final lap, but allowed Whincup back through for fifth at the death.
The squabble allowed Mostert to drive into the distance, with the 29-year-old cruising to victory by 10.2s over De Pasquale.
Davison clinched a second podium in as many starts, with Waters fourth for the second race in succession.
WATCH: LAST LAP DRAMA
Goddard scored a career-best finish of seventh ahead of Penrite Racing’s David Reynolds, who survived earlier contact and a rubbing tyre to finish eighth.
Tickford’s James Courtney and Brad Jones Racing’s Nick Percat rounded out the top 10.
Mostert also clinched the five bonus points for the fastest lap of the race, as he did in Race 7, with a 51.7604s set on lap 17.
The result also ended Red Bull Ampol Racing’s seven-race win streak to open the 2021 season.
WATCH: MOSTERT SHREDS IT UP
“Yeah look, it’s been a long time for the first place for us in Supercars, and me personally,” said Mostert, who is 165 points behind championship leader van Gisbergen.
“Starting to believe I couldn’t do it for a long time. Super stoked for the team.
“We were there and thereabouts on pace all weekend, but that car in that last race was a lot of fun in clear air.
“I’m so happy to finally get the win and get the monkey off our back.”
Van Gisbergen’s championship lead over Whincup has been slashed to 139 points ahead of next month’s OTR SuperSprint at The Bend Motorsport Park.