Flatpack furniture sells because it's cheap and generally easy to transport. If you find that you've purchased flatpack gear that won't go in your car, consider unpacking it instore.
I hit this exact scenario recently when buying some cheap bookshelves. I had measured the shelves in-store, and I knew how wide and long my car's boot was.
There was one factor I had failed to take into consideration, however. The shelves themselves would fit in my car, but flatpack furniture never comes bare; it's often packaged in quite a lot of styrofoam and cardboard to make it easier to ship from the factories where it's put together.
Upon grabbing the boxes from the loading dock of the store in question, it became apparent that I was out on my measurements, gallingly by less than a centimetre. I could have dug into my wallet and paid for home delivery, but that would have cost nearly as much as the bookshelves did, which seemed like a waste of money.
So instead, I asked the store if I could unpack it then and there. This they had no problem with, and the loading dock had a recycling bin nearby that I could dump the cardboard into, which seemed ecologically sound as well as saving me the trouble of doing so later.
There are some inherent risks with this approach. You've got to pack carefully, because the exposed bits of your flatpack furniture could damage your vehicle, and you do assume some risk with damaging the furniture yourself as a result.
That being said, it's also a good way to spot any flaws in the product before getting it home, and in any case if there were larger structural flaws you don't need the packaging to secure a refund or replacement under Australian consumer law.