Stef92,
you have definitely got this wrong.. this would be a fraud and it could happen anywhere. Australian and Californian or even Spanish and Greek not only have different varieties of olives and therefore oils, but they are bigger producers and therefore tend to be cheaper, however, there are some brands which are very expensive.
The quality of the oil depends on: quality of the olive and variety, not all varieties can produce good oil, ripeness of the olives, conditions of the olives when they reach the mill, they should not be damaged, and pressed asap from the harvesting. The other difference is made by the mill, there are plenty of mills around but new technology allow producers to enter the olives and get the oil, benefits are that the oil stay protected from the oxygen, oxidation problems wont happen, and nitrogen is used for its conservation instead of oxygen again to protect the quality. Dark bottle is a must.