If you want to rank well in search, where a regional competition, which tend to be hosted on a server in this field, or if you have a top-level domain, which belongs to the region, or both.
To target for USA, there's nothing to change, it's just the same with targeting global. For Australia, you can do geo targeting for it, by submitting you site to sites that is based in Autralia.
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Well,Having your website listed on the first page of search engines will give you an answer for maximum exposure. The closer you are to the top of the page or position the more you have in search results, plus the number of visitors you receive.
I think no need for special treatment for USA and au site.Same SEO process you can use.If you are doing local optimization then it will little bit different otherwise no need to apply any different process.
Its right that We can get various result at different country. Google really provide local search engine result. Most of the user do SEO in local area. So they will get more back links as far and also increase their prestige in their local area.
As per mine own experienced, You have to optimize your sites at local area. After You have to promote at global level. You have to optimize your site for SEO. Article writing and press releases are useful which increase our PR.