Hi. Sorry to see this happening. This isn't an orchid that I've crossed paths with before, but the black spot I have. I think it's a fungus. If you use some physan 20 or some have used a really weak solution of bleach and water. Remove all potting material and start over with all new stuff, potting mix, clean potting area, tools, use gloves. It could spread. It may have just been waiting there for an opening to sneak in. I may also suggest, as painful as it may be that you amputate, if there are viable roots for the rest of the plant to survive.
You may not have to if you try something I've found to work sometimes, the physan or bleach water bath then put it to dry for a few days nekked. Yup, no pot, no potting mix, no nothing. A nice lightly humid but not damp area away from prying eyes and paparazzi on a clean sheet of paper. Fungus can grow if there's enough moisture. It may take a light spray of water on the roots only if they look dehydrated in a few days. A week or two usually works for me, but I have a more humid environment. Just keep an eye on the fungus growth and the roots. Keep the moisture to a minimum
Just note that this may just be bad luck on your part. Trust me, I live on a 166 square mile island, a speck in comparison to Australia, but we have vastly differing micro climates. What was perfect for my best friend in Christ Church, was horrors for me in St. Michael. A twenty minute drive away. Plus the seller you got it from may be in a walled in apartment and you in wooden house. You may have more trees and a lower elevation to the sea, but they the same amount of trees but it's dryer. It's probably just adjustment. Give it some time.
Hope that helps. Keep us posted.
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Ooh, I'm such a ninny. I forgot the obvious. Where is it situated? Was it getting a lot of sun before? If not and it was sitting in an area where it's getting more that it's used to it could be sunburn. Still use the method I mentioned to prevent a fungus. As I said before it needs to adjust. Give it time.