A bogan is a steriotypical Australian, with a potty mouth, bad dress sense & a fondness for alcoholic beverages - note the esky next to him, no doubt full of beer or other alcoholic beverages.
So, Nyngan is situated along side the Bogan River, hence they came up with the idea with a bit of a play on words, of a BIG BOGAN as a bit of a touristy thing/talking point....and Aussie towns do seem to love "big" things as tourist attractions (big pineapple, big merino, big banana, big prawn etc etc).
So here folks, you have the BIG BOGAN in all his glory! He stands about 6 metres tall & weighs 1500 kg 😮
