Another welcome to FT. The only additions to Kiwi Flyer's excellent response is that you have some flights that can be purchased cheaply outside of the ONEx ticket such as MEL-SYD and NRT-NGO. This will give you more segments to use which would otherwise be expensive. As for where to buy the ticket, another possibility is GIB. The agency there (Bland's travel I think) are well versed in ONEx and dealing with BA.

Minor edit to number_6's post. It is 2 stopovers in continent of origin, not country.