When you want to trade as something other than your company's legal name you will need to register a business name.
If you register a company called 'ABC PTY LTD' then you can trade Australia wide as that name. You do not need to register a business name. However if you want to be able to trade as 'ABC Plumbing' then you must first register 'ABC Plumbing' as a business name belonging to 'ABC PTY LTD'. Finally if you want to be able to trade as just 'ABC', ie without the 'PTY LTD' then you will also have to register 'ABC' as a business name.
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