Individuals involved in any type of business should make sure that they can fulfil all their tax obligations. They have to keep accurate financial records and register for taxes that indicate their business activities.
Otherwise, they may not only pay an incorrect amount of tax and suffer penalties, they may also be unable to claim the deductions and credits they are entitled to.
According to the IRD, being in business pertains to the regular activity of providing goods and services with the intention of making profit. If your job fits the description, then you are in business, and thus required to meet some tax obligations. This includes a tax and GST registration.