xONEx tickets are sold and priced based on the ORIGINATING airport. Although you can buy in Australia and pay the Australian prices, most FT members would question your financial sense in doing so.
If you want the South African prices you must have the first ticket (segment) depart from South Africa. There are a number of cheap/inexpensive ways to get from Australia to South Africa BEFORE you commence the xONEx ticket.
Happy wandering
Fred