You will be assessed to pay tax on income above $18k if I remember correctly. That said on lower levels of income it's quite likely you're expenses will neutralise any tax obligations. E.g. Keep records of costs such as maintaining a home office, driving costs, phone internet etc. Id say get an accountant but if this is your total work for the year you might want to put the time in reading information on e-tax. Make sure you read about deductibles before you start now so you can keep records.

i'm no accountant but i'm pretty sure sole trader would be paying personal income tax rather than company tax, which would be for registered companies.