If you do a search for "bulk billing doctors Melbourne", you'll notice the section Google shows that lists businesses. Not the ads or the organic search results, but the section which often includes a map. In this section you'll notice review ratings along with the term Medical Center. Yes the American spelling.
I've noticed this in other online services, such as LinkedIn, where you'll find the occasional American spelling.
Whilst it may be easily tricked and forget the Australian spelling, just remind yourself you're using software or services that have been developed and supplied by American companies. The companies should take the time and effort to localise their software or service, but their efforts in those areas is often lacking. We're just not important enough to them.
If you do get confused then keep in mind you can use Word Check to check if you're using the preferred Australian English spelling. Word Check is available at www.australian-dictionary.com.au/wordcheck/.
The preferred Australian English spelling.