Jay, you are getting mixed up between the limits for domestic and international.
The allowance for domestic is different from international. For domestic, it is a piece allowance where if you are travelling economy, you get one piece which can weigh up to 23kg (assuming you are not Qantas Club or a frequent flyer). You get charged for every additional piece of luggage so if you have two pieces which weigh 10kg in total, they can charge you for the second piece.
For international, it is based on weight. There is no restriction on the number of pieces of the luggage but you are limited by weight. For an economy customer, that means you can have as many pieces as you want BUT the limit is 23kg.