From the USPTO announcement:

Effective November 1, 2008, the Australian Patent Office (IP Australia) will act as an International Searching Authority (ISA) under the Patent Cooperation Treaty for international applications filed with the United States Patent and Trademark Office (USPTO) as a receiving Office, provided that (l) the applications are submitted in the English language, (2) the applications do not contain one or more claims relating to mechanical engineering or analogous fields of technology as defined by certain International Patent Classification classes, and (3) IP Australia is chosen as a competent authority by the applicants of said applications. IP Australia will also act as an International Preliminary Examining Authority if these three requirements are met, and IP Australia acted as the ISA.

The USPTO will collect the search fees from the applicants, and will transmit the search fees to IP Australia. The search fee for IP Australia acting as an ISA for international applications received by the USPTO is $1,514, effective November 1, 2008. Any other fees necessary for search or examination or incidental thereto will be paid by applicants directly to IP Australia.

The $1514 cost is less than when choosing the USPTO or EPO, but significantly more than the KIPO. Of course, this is to be expected based on the relative economic strength of each country, and the fact that the USPTO needs to charge enough to cover the cost of paying and managing their contractors, while still having enough left over to line the pockets of Jon W. Dudas.