Beyond the caching issue... you should consider changing the server location to help with geo targeting when you change the IP for the AU site. In determining geo area, Google looks at cc TLDs, geo-location declaration in Webmaster Tools, language, onsite content providing localization clues, geo-sources of inbound links, and the geographic location of the server.
Assuming from your first post that you are currently hosting both sites in New Zealand... if you can find satisfactory hosting at a good price for your AU site in Australia, that's likely to help Google with the geo-location, particularly since both sites are in English.
I don't know whether the content of these sites is otherwise differentiated, or where your current inbound links are from, nor do I know how much Google might otherwise tend to lump Australia together with New Zealand, but I think hosting the AU site in Australia could well help you rank in Australia. You should also get inbound links to your AU site from other Australian sites.
In the following video, Matt Cutts isn't emphatic about it, but, as I read the video, it's a recommendation to host in the geo-targeted country if you can....
Can the geographic location of a web server affect SEO?