Since wireless regulations vary by country, you might experience compatibility issues if you try to use units manufactured for the USA in Australia, and Google will be unable to offer support.
Technically speaking, USA units use a 5 GHz channel that may not be allowed in Australia. They should still communicate with each other, and some client devices will be able to see this channel and join at 5 GHz, but some clients won't. In particular, mobile phones are a good example of a device that's likely to be more stringent about adhering to wireless regulations depending on the country they find themselves in. In essence, it's likely they will still work, but may join at 2.4 GHz instead of 5 GHz, so may not be quite as fast.
