Relies upon your part, and how pervasive it is at the area to which you'd jump at the chance to exchange.
Illustration #1: If you're a product design whose execution is on favorable terms, it's in reality quite simple to exchange to any of Google's overall building focuses (of which SYD is one). Visa accessibility and accessible headcount are regularly the gating factors.
Case #2: conversely, in case you're an electrical specialist working at one of Google's server farms in the US, you wouldn't have the capacity to exchange to Google's Sydney office (which isn't a server farm), as there likely wouldn't be a part fit for you.
Note: regardless of whether Google will *pay* for your migration is another inquiry, and is affected by whether the change is organization started or worker started.