What? Why not?
If a plane is going at a hundred miles per hour (and in Australia it looks distorted by a factor of three) and you go three hundred miles in one hour, I think somebody's going to notice, even if they can't see the difference.
You've got it backwards. Seeing the difference is the only thing that happens, and only from sufficient distances. Mechanics != optics.
I don't understand! What I'm saying is, people have clocks, and one hour is noticeably different from three hours. Sure, bendy light tells them, "Nope, three hundred miles, not one hundred (as it really is due to Earth's flatness)." But their plane scheduler, and their pilot, etc., (believing there to be three hundred miles as well) will say, "We have three hundred miles to go. We are currently going at one hundred miles per hour, so we expect to get there within three hours."
Twenty minutes later (a third of an hour), the pilot will realize that they've gone a full hundred miles! But the whole time they've been watching, and they've never exceeded one hundred miles per hoour! By this standard, they would have had to be going three hundred miles per hour! But the pilot checks the reports, nope, one hundred the whole time! As this trend continues (because, you know, the world's flat), he will notice it more and more. He'll think, "Well, I guess there's something wrong with the equipment. I must be going at three hundred miles per hour." Surely he'll anounce to the passengers, "Well, we seem to be having some strange technical disruptions, and it seems that we're going at three hundred miles per hour, not one hundred. We will be there much sooner than expected." This goes against the pilot's intuition, though, because three hundred miles per hour is ridiculously fast. The pilot will likely make a report to whoever's in charge about this.
You may say, "Well, they are in denial, to avoid changing their whole world view." Well sure, but there are flights over Australia every day! Surely the director or whatever will continually have the equipment looked at, but there will be no problems (because the real problem is bendy light). Eventually they'll start to wonder. Besides, "flat Earth" never occurs to them; that is patently ridiculous. They have flights over Antarctica all the time. They won't have a reason to deny this, because they don't think it will cause a scientific upheaval; they just think it's so weird and seemingly unexplainable, but never ceasing to show itself.
Also! Crops in Australian outback! You never explained that!