There are various species of wood called mahogany and the quality of it varies a lot too. I have two yachts with mahogany. One has a mahogany hull, the other only a mahogany transom. Both have had rot in the mahogany but in both cases it's where rainwater has been able to saturate wood or plywood that sits against the mahogany. If your deck is really leak proof, especially around the gunwales, and you don't have areas where water can be trapped against the mahogany, then I think you'll be fine. There are many boats built from mahogany in Australia and many have lasted well. You will need to use anti foul suitable for Australia as we do have serious problems with teredo and other shipworms, although I don't think ours are quite as voracious as some in the US and Carribean. Good luck - would love to see some pictures of your boat.
Rick