Australian Production Car racing spread like a highly contagious virus during the mid/late 90's into the new millennium.
What started out as one class with the top blokes all trying to out smart/spend each other, morphed into three seperate classes: GT Production, GT Performance & Nations Cup with regulations regarding allowable modifications being more relaxed as you moved up the chain. Both GT Prod & GT Perf started out sharing the track but with numbers exploding, they got their own races like Nations Cup except for the Bathurst 3hr that ran as a support race to the Supercar 1000km enduro.
As for the exact regulations covering Nations Cup, PROCAR recognised that, in the name of competition, some cars needed more concessions/freedoms than others to have a fighting chance.
Porsche started out using the 911 RS/CS (993), the Vipers were essentially the '99 GTS as found in GT6 & Lamborghini used the Diablo SV. These eventually were replaced by the 911 Cup, Viper ACR & Diablo GTR, spending got out of control, GRM built the 427 Monaros, dominated the Bathurst 24hrs upsetting everyone & PROCAR walked away.