Most (if not all) car rental agencies I've encountered living in Australia add additional drivers as standard for free (so long as certain criteria are met). Though I've mostly used these for hiring cars in Australia. For example, if I am going to the Gold Coast, both me and my partner are able to drive for no extra cost.
This does not seem to be the case though outside of Australia, but rental websites seem to be unaware. For instance, a few years ago we travelled to Manchester, UK. Despite the rental agreement (booked through Avis.com.au) saying we had an additional driver for free, the agents at the airport would not honour it (and said we can either pay the up-front costs, go without, or not rent through them and they'd refund us as soon as they could). In the end, in this case, we paid the extra fee (which was higher due to being in a different currency).
I've just quickly been through Avis.com.au again and nowhere in the reservation process is there an option to pay for an extra driver. So how are we supposed to do this via Avis.com.au (and other similar car rental agencies with an Australian website) without having to pay a hefty, unknown amount at the other end?