That's a rather selective reading of what was said.
There are companies that offer unlimited mileage rentals in Australia and in many of the major cities. That doesn't mean that every rental is this way. You have to read the fine print of the rental agreement to make sure you don't get yourself in over your head.

Insurance for your travels, medical & car rental, are something to research along the way too. You'll want to rent from a reputable company with locations generally near the route stops you're planning on. Cars breakdown and the last thing you want to do is waste time on repairs. More prevalent rental companies can replace the car quickly to get you back on the road.

Driving on the "other" side of the road takes a little getting used to but isn't that big of a deal. One thing to make sure is that you rent an automatic transmission vehicle as that's one less thing to worry about while driving.