Hi! There is a 21 step setup guide for gtr2 which helped me a lot. Just google it. Although this is rfactor the basics are the same... Take a look at it.
For example, at bathurst the backstraight has a slight turn right, i used to take 8 or 9 values to rear wing and i had too much understeer, so braking to the next corner was hard. Right now i run my setup at 2 points in the rear wing but for a long time i placed it at 1. Thats one point, Aerodynamics.
Then there's the surface of the road, at that specific turn i had to set fast bump and rebound with high values, for the only reason that there is a high bump inside that turn (which is my racing line) and i mustn't lose grip there so a i can brake safely to the next turn.
What i'm trying to say is it's hard to help with so little information. I love bathurst and seriously i've gone around it about 5000 times, laps, and only last week i was able to run below the 2:09 second...
Change values within the rear wing and suspension but first try go round it with 5th gear and see if u get under or oversteer, 4 wheel drift, does it loose grip mid corner, or is entering in that corner hard, ony then you'll be able to get better at it. Go in and change values and run 5 or 10 laps and see how it goes.
These cars use tires which the lvl of grip is at its best at around 100ºC... That will help a lot, it's a pointer that your getting the most grip out the car so the info the car gives you is more reliable.
I'm not a master of setups but this is what i do. Cheers