My understanding is that in the USA you can't buy a "fully-automatic rifle", which is what the rest of the world calls a machine gun. Nor can you buy an "automatic weapon" which is a gun that looks like an automatic rifle. You can buy a semi-automatic AR-15, which can shoot 45 rounds or so per minute.
As an Aussie, I'm also cool with not allowing people to have AR-15s. A round or two a minute should be enough for most people.
The terms are not confusing. "Assault rifle" is a technical term with a precise definition. How a gun looks doesn't actually matter.
Maybe in your country. The Encyclopedia Britannica says:
"assault rifles are distinguished by their high rate of automatic or semiautomatic fire...In those countries where assault rifles can be purchased in the civilian market, their sale is subject to various restrictions, such as the elimination of automatic action and of the capacity to fire high-performance military ammunition."
So an assault rifle can be automatic or semiautomatic, and you may buy an assault rifle that has been modified to remove automatic action. Isn't that confusing?
What you're probably thinking of is "automatic".
Civilians in the US are not allowed to own assault rifles either, and haven't for decades. There are a tiny number of assault rifles in civilians' hands that were first sold over 30 years ago and were grandfathered in, but they are absurdly expensive (due to their rarity) and are generally owned by collectors.