I'd suggest something else... maybe a lion, to go along with the fern and 'roo? The crosses just don't look so good - dark colour on dark colour...Well, it more represents the British (Look at the Union Jack) heritage/history on the islands rather than whites in general.
Is this a modern union, or historical? If modern, I'd use this for the aborigines (it seems that they would): http://en.wikipedia.org/wiki/File:Australian_Aboriginal_Flag.svg