That means he is essentially only a mathematical chance of winning the title in Newcastle later this month, with 300 available across the two races.
Mostert admitted the troubles were spread across him and the car at Pukekohe.
“We struggled with the car most of the weekend and, actually, in that last race I think we got on top of what we had,” he said.
“We had a fair few braking issues and we kept thinking it was the car but it was actually something else.
“In that last race, it was disappointing to be in a lap one incident that put us back, obviously [on Saturday] struggling with the braking-type stuff I made a balls-up trying to get as many points as I could.
“Not a very good weekend from a driving point of view for me, also we struggled a bit.
“The pace is obviously in the car, [on Sunday] we showed it can be at the front and at the pointy end.
“So it was disappointing not to get some champagne for the Supercheap Auto guys, the car they gave me today definitely deserved it.”