Even though the rolling stock is technically more advanced and loaded with sensors (sometimes resulting is degraded service aka. lower speed for security reason

Even though the rolling stock is technically more advanced and loaded with sensors (sometimes resulting is degraded service aka. lower speed for security reasons) the main cost definitely lies in infrastructure. You basically need to rebuild the whole track for highspeed trains. And with Australia being a litigious nation it will also take a while to acquire and execute the land rights.