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If I understand correctly, the American branch is building an entirely new website, and in the meantime they want to direct any traffic that comes through their domain to a landing page at com.au. Is that correct? Will there will be any negative consequences to your .au site? In most cases, no, and this might even make perfect business sense if the parent company is the most appropriate place to direct people. If the American .com has a clean backlink profile, then directing any traffic to the .com.au site should be fine. Unfortunately, without looking at the sites in question there are a lot of unknown variables and it would be almost impossible to account for them all in the space of this answer. In the end, I think you should be fine, but in the case that, you do experience any negative consequences, simply reverse the change and you should be okay