The frequently asked questions wiki page on this subreddit has a section dealing with Gavin Menzies and 1421. As you'll notice if you read the posts linked to in that FAQ, historians do not have a high opinion of Menzies. Also, an excellent blog post by /u/mikedash - a quality contributor here - goes into quite a lot of detail on the trade networks that existed between China and northern Australia, and you may find that illuminating.