A colloquial slang term taught to Australian children by their parents, especially in New South Wales. It refers to the female human genitalia in the same way as 'willy' refers to the male parts. It is often shortened to 'wiki', which is why many Australians thought Wikipedia was a humerous name when it first appeared.
Child to mother: 'Tommy wanted to see my wiki-wocka today at school, but I wouldn't show him'.
Mother to child: 'You're a very good girl'.
Sorry to be a bit dull with this definition, but it's a touchy subject for some, and I don't wish to offend.