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Taxable income on full-time job + business earnings
In Australia, any income you earn is taxable despite where it came from. Using your example your taxable income is $70,000. Keep in mind that with a business even as a sole trader any business expenses that contribute to the earning of your business income is deductible, reducing the final amount of tax you'll have to pay.  The ATO website has lots of good information and examples to look at including tax rates.  If your total income is pushing  into a higher tax bracket over 30c tax per $1 earned, it may be worth looking at shifting your business to operate under a company structure that just has a fixed tax rate around 30c per $1.  That said, for me, I don't want the paperwork overhead of a company yet so I'm running my side business as a sole trader too. I'd rather do that and keep it easy for now while my business gets profitable that waste time on admin structures for tax reasons even if in the shortterm it may mean slightly higher tax. In the end, you only pay tax on profit (income minus expenses) as opposed to raw/gross income.  For more info there are good books in the bookshops or local library (to read free) on starting a business on the side while still working. They discuss these issues too.