The good, the bad, and the ugly
Australia is basically 4,000 Km long and wide. I'm basically 1.75 meters tall and 0.3 meters thick. That's a ratio (as good as any) of 0.17, so I'm proposing that your creature is at least 680 Km thick (making it a tapeworm, not a snake).
Earth's atmosphere is 480 Km thick. But most of it (e.g. 50%+) is in the first 16 Km.
The FAA mandates a minimum of 1,000 ft above everything in a 2,000 ft radius. Granted, that's for airplanes, which are less than 1% of the size of this critter. (A lot less, actually, but let's roll with 1%.) That means your critter needs a minimum of 30.5 Km. That's not very much, considering it's 680 Km thick, a burp may cause it scuff the ground.
Final number: the top of the creature is 230.5 Km into space. That's inside the range of low orbit. The bottom line is your critter probably has a snorkle descending into the lower atmosphere through which it breathes, and typhoon-force winds happen around the opening of the snorkle. Heaven help the good folks underneath it when the critter sneezes.
That was the good, now let's talk about the bad
I haven't done the math, but I expect the mass of this creature given its proximity to the planet will cause tidal changes and tectonic shifts. The world will literally reach up to the creature. Probably only by feet (maybe a dozen or so), but it'll be enough to fracture fault lines, cause minor eruptions, change currents, and basically make life on the planet itself something of a hassle. You said in a comment that you didn't expect life to exist on the "floor," so this might not be as bad as it sounds.
Aaaaaand the ugly
The problem is that the creature won't have enough mass to generate significant gravity. Especially in comparison to the planet below it. Since the top of the creature is in low orbit, you can't build anything on its back unless you have dome technology and can mine oxygen from its skin (mining the creature would be big business, btw... humanity being less pestiferous than bacteria is to humanity). And you can't build on the bottom, either, since the planetary gravity will simply pull you to the ground.
You would, however, have cliff-dwelling birds making nests in every fold of skin or everywhere they can make a bit of mud stick.
Creatures of this size tend to be very impractical unless you're in the mood for full fantasy — in which case you simply declare it to be so and have fun. (I love Terry Pratchett's work, BTW.)