OK - there is a dish / plate that looks very like yours on the Powerhouse web site
http://www.powerhousemuseum.com/collection/database/?irn=111237
but it is unmarked and the attribution comes from the owner who donated it, along with 2 others (which are not at all similar.) The collection donated by Dr Robert J Lyons was most significant - but unlike other items of his which I can find on the PH web site, these leaf dishes have no references or collection notes to aid attribution.
I have several books, and like you have Googled extensively but I can't find any piece of Lithgow pottery that looks like this. While it is true that Lithgow did not mark all their pieces, in marking they were well ahead of any other contemporary Australian pottery manufacturer. The two leaf pattern pieces that that Marjorie Graham illustrates in her book both have the kangaroo mark. Also the colours don't look quite right to me for Lithgow but I am not an expert.
I would call it a majolica leaf dish / plate in the begonia pattern. If you google that you will find many similar in style. Of course you would be quite entitled to put Lithgow? with a link to the very similar plate in the PH collection (and that is probably what I would do if I were selling it.) There are of course always quite a few unmarked items around that claim to be Lithgow - for obvious reasons they do not bring as much as marked pieces. And the market is fraction of what it was 25 years ago, both in terms of collectors and prices achieved. But if it is indeed the real deal, I am sure someone will spot it. Hopefully someone more knowledgeable will come along.
