A "good cunt" is a good person, top bloke, someone you want to be around. It's the exact opposite of a "shit cunt" (for the uneducated cunts, replace the positive adjective with shit or other derogatory type word)
One of the highest ranks given to a mate in Australian culture, if you are called this it means you’re apart of the elite group of dead set good cunts who make life’s worth living.
Oi did you see that bloke do a helping that old lady across the street, dead set good cunt.