Art Gallery NSW and Restorer
I finally managed to get Barry Pearce, curator of Australian Paintings at the AGNSW to have a look. His initial reactions involved a lot of chin scratching and head shaking. He was concerned about how small the signature was, and that the painting appeared to be coated with a very thick varnish - unusual for any painting. He also was concerned about the level of detail in the painting - that Drysdale was 'not concerned with detail'. In a way I was glad to hear this because this was one of my concerns when I first inspected the painting.
Although he noted that there were some arguments for it being a Drysdale (palette, plausible signature) his theory is that someone had found this painting, thought that it looked a little bit like a Drysdale so added the signature, then covered it in a thick coat of varnish so that tests with 'blue' or 'black' lights would not be able to discern that the signature was added long after the painting was completed.
However, he did acknowledge that this was just a theory, and that the only way to come to any definitive conclusion would be through forensics. He recommended that I take the painting to, a trusted restorer, to remove the layers of varnish over the signature area and then run the 'blue' and 'black' light tests.
So the painting is now at the restorer awaiting this treatment...