There's no sure slot for Australia. With the two Koreas, Japan and the Middle East cluster, Australia still has to fight for it. With Australia having left Oceania, the 0.5 slot for NZ is practically assured.
Australia left Oceania because they're sick and tired of playing with little Pacific islands; you can't improve much that way. There may be some geographical logic to join South America too, the highest qualification ratio given by FIFA at almost 50%, i.e. about half of SA countries would qualify. However, that's also the toughest qualifying zone in the world with all-in-one qualifying league instead of groups. There's no way of escaping Brazil and Argentina through the luck of the lots; must face both. Oceania too soft, SA too tough, so naturally chose to join Asia.