Are there many Australians that use these essay writing company/writers

I work with many AU clients. I am a native US writer, but I pay close attention to the literary and academic rules of every client's respective country.
Since I am American, I've not tried to work with companies outside the US. When I was freelancing with companies, I only worked with a couple reputable ones in the US. I am particular about the companies I associate with as that can impact my reputation as much as my independent services.
As such, I can't say there are or are not AU writing companies nor the quality of AU companies.
However, if you align with a good US company or independent writer who frequently assists AU clients, you should be fine. Just be sure to specify what you need upfront (i.e. British English, specific citation formats, etc.).
If you don't note these upfront and later need changes (such as changing from American to British English or to Harvard from APA), you could run into additional fees.