Google's Chrome browser generally defaults to determining your location using your IP address and customising its search domain using that information (so Australians usually see results via .com.au). If you want your searches directed through a different Google domain, you can change that using a simple settings edit.
Potential reasons for wanting to change the browser's behaviour might be if you prefer using the default .com interface (which generally gets new features added faster), or if you're overseas but want results oriented to your home country or language. Either way, as Google Operating System points out, you can alter it by opening the User Profile directory in chrome, editing the Local State file, and changing two variables to include the country code you want.
Hit Google Operating System's post for more detail. That fix is Chrome-specific. If you want to change the default search engine in other browsers, check out our guides to how to change the default search provider and how to get Australian search results in Firefox.
Change Google's Search Domain in Google Chrome [Google Operating System]