Sounds like the same moral quandary: getting a .com.au registered domain name by a not so legal path or registering for an ABN to make it legal.
The reason the ABN requirement exists is to ensure the .com.au domain truly represents (as best it can) a registered Australian commercial enterprise, unlike .com which any Tom, Dick or Harry can get hold of (like me for one!)
The Australian government doesn't really give a crap who registers a .com.au domain, those requirements are stipulated by auDA, a private body responsible for .au policies. Lying to a private organisation which has no legislative powers isn't illegal*. Some might call it immoral, but I don't. At the end of the day, I wanted a .com.au for personal and legal purposes, so I got one that didn't involve lying to the Government or breaking any laws.
Had I have registered for an ABN knowing full-well that I wasn't going to be conducting business could have had other implications for me down the track.
I know the reason why the rules exist, but in reality, it offers minimal consumer protection and confidence at best and they are easy to circumvent. I believe that a .com.au is no different to a .com other than it's Australia-focused.
Also I have no issues providing private organisations with fabricated details. My domain registrar doesn't need to know where I live or what my phone number is, they can contact me via e-mail if the need arises. I don't even give Ikea my correct post code when I shop there.
* Lying to your bank or insurance company is different. Don't lie to them.