I read she's using her Australian accent, but it sounds like it floats between Australian, American and British.
For example, she says passport like an American. Australians say it as Pahs-port.
I'd say she just sounds like an Australian actor who's lived in America for a long time.
I have to admit, I didn’t catch on that she was playing an Australian until Sarah Paulson’s character mentions it, it certainly did seem like a hybrid accent. Which I guess makes sense since it was established that she had quite a long history with Debbie, her character had obviously been living in America for a while.
For me it was her American pronunciation of bathroom that seemed the most jarring.