It was a belated win of sorts, with Mostert leading late on in the Sandown opener before he was demoted to sixth amid a tyre life struggle.
It came after a strong run at Bathurst, with Mostert scoring two podiums behind the dominant Shane van Gisbergen.
WATCH: MOSTERT DOMINANT: RACE 8 RECAP
To next month's event at The Bend Motorsport Park, van Gisbergen leads teammate Jamie Whincup by 139 points, with Mostert just 26 behind Whincup.
It's a greater margin in the teams' standings, with Red Bull Ampol Racing 506 points ahead of the Shell V-Power Racing Team, with WAU six points adrift in third.
The field closed the gap to the Red Bull Ampol entries in Tasmania, despite the team scoring consecutive one-two finishes to open the weekend.
As Shane van Gisbergen set a record pace in Race 6 qualifying on Saturday, positions two through eight were covered by just 0.1s, with Mostert lining up third.
Mostert then managed just 10th in Race 7 qualifying, but was a mere 0.2s off pole.
He improved to second in Race 8 qualifying, and was just 0.018s off Cameron Waters, who swept the Sunday poles.
Mostert was dominant after winning the Race 8 start
Having been on the fringes of victory in his opening 35 races in WAU colours, Mostert acknowledged it seemed only a matter of time before he hit the front.
"I think if you look at the last three rounds, we’ve been there and thereabouts," he said.
"If it’s through qualifying trim, or races. Bathurst, rolled out pretty good for us, got a couple of trophies there.
"Look, we’re there and thereabouts to get stuck into them. If we can beat them week in, week out, that’s the next thing we’ve got to keep working towards.
WATCH: MOSTERT DISCUSSES WAU WIN
"[Qualifying] 10th, it shows how tight it is, how cutthroat it is at the moment."
Aside from his Race 5 throttle issue in Sandown, Mostert has finished no lower than sixth in 2021.
However, that single race cost him crucial ground in the title fight; in one race alone, Mostert dropped 78 points to title rival van Gisbergen, who raced away to victory.
The same went for Waters, who haemorrhaged 105 points to van Gisbergen in the Mount Panorama opener following a power steering issue.
Mostert was untouchable in Race 8
While claiming his Race 8 win was proof his #25 Mobil 1 Appliances Online Commodore ZB has pace, Mostert admitted his runaway win may have skewed his weekend’s data following finishes of fourth and sixth.
"We need to keep pushing forward, we need to keep looking for that next step,” he continued.
"That last race was a good step for us in the set-up direction.
"But it can also skew the data; once you get some clear track in this category at the moment, your car can look really good.
WATCH: MOSTERT EMOTIONAL AFTER BREAKTHROUGH WIN
"If you’re behind someone, you don’t really know where you’re at. We’ll just have to go through the data and chip forward.”
Mostert won the start in Race 8, with Waters battling with tyres late before falling in the clutches of the Shell V-Power Racing Team duo.
It allowed Mostert to race away to a 10.2s win, which was the biggest margin of victory in a full dry race so far this season.
Whincup and van Gisbergen fought to fifth and sixth respectively amid their own tyre battles, failing to overcome Waters for fourth.
Mostert and WAU celebrate after Race 8
However, Mostert didn’t read too much into his winning gap, confident the margins in the field will remain tight heading to The Bend next month.
“I wish it was more to be honest,” the 2014 Bathurst 1000 winner grinned.
“Obviously the car was pretty good; if you look at Cam’s performance across the weekend, he was obviously struggling a bit in the races through tyre life.
“I think he banked [Anton De Pasquale and Will Davison] up a fair bit, and that’s why the gap blew out to what it was.
WATCH: MOSTERT’S RIPPER SKID
“If these guys got past Cam earlier, the gap mightn’t have been too much, I’m not sure.
“It’s no different to the T8 guys in the first couple of races streaking away; it’s a clear track gain for the front car.”
Mostert trails van Gisbergen by 165 points heading to the OTR SuperSprint across May 8-9.
The event will be broadcast live on Foxtel and will be streamed on Kayo with highlights on 7 and 7 Plus.