If you already have an IRD number for non-business income and you are a sole trader, you use the same number for your business. You would also use this number if, as a sole trader, you need to register either for GST or as an employer.
If you are going to operate your business as a company, partnership or trust, you will need to get a separate IRD number. See Government: Paying Tax for information on getting an IRD number.
