@kOtZ @maguero I have my middle name on a previous AU tourist visa (that a relative applied for). They've put it on the given name field and it didn't matter. I got in without problems and spent a month here. I have various other tourist visas and boarding passes with and without the middle name on them.
My 189 visa and all AU government forms/IDs don't have my middle name. The automated ID checks fail (like opening bank accounts or new phone plans) if I put my middle name in since the visa doesn't have it, so for consistency I match everything to the name on the visa.
Most AU forms require last names only since not everyone in AU has middle names. There's also things like preferred names (someone usually known as "Dick" on all IDs but legally "Richard" on the birth cert) and mononyms (one-word names like "Beyonce"). The government has to accommodate all of these.
The only form I've seen where the middle name is absolutely required if you have it is on the citizenship application, I haven't seen anything else yet that required it.