It's salt, but you're right, 21 feet would be about as big as Great Whites get....
This one was caught in a set-net in Seven Star Lake, Hualien County, Taiwan, on May 14, 1997. Gross weight of this animal was 2500 kilograms (5,500 pounds) and total length is estimated(?) at 6.7 to 7.0 metres (~22 feet). Possibly the largest great white shark ever recorded.
I suspect this might be a case of "you should have seen the one that got away! It was this big!"
Of course there could very well be a Whitey in Tuggerah Lake, remember the 2m baby white caught a few years ago? Maybe Mum's come lookin for baby?