Herne went into the second race of the Adelaide weekend unbeaten from the four Tasman Series starts so far.
However his winning streak came to an unceremonious end when he slowed at the end of the opening lap with a mechanical problem while chasing early leader Mawson.
He managed to circulate slowly to be classified 11th but, with Mawson streaking to victory, Herne's comfortable series lead has been reduced to a single point.
The Tasman Series will be decided by the final race on the streets of Adelaide tomorrow with 50 points up for grabs.
There was confusion before the start of Race 2 with some in the field mistaking the formation lap for a first racing lap.
That led to Mark Rosser and Brad Gartner making contact at Turn 7 with Rosser ending up in the wall.
Elly Morrow, who was right behind the pair, spun to avoid Rosser but managed to keep going.
The drama continued when the race finally did get going with Mawson jumping Herne on the run to the first corner to take the lead.
When Herne's chase ended with that issue on Lap 1 Mawson was clear to take a first win of the Tasman Series campaign ahead of Cooper Webster.
Formula 1 star Giancarlo Fisichella was running third until Lap 6 when he suffered a broken rear wishbone.
That promoted Blake Purdie to the final spot on the podium.
Jordan Boys came home fourth in a tight battle with Ben Bargwanna and Aaron Cameron.
Noah Sands, Jude Bargwanna and Gartner rounded out the Top 10.
The series finale kicks off at 10:10am local time tomorrow.
