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Investing tax (savings)
If you have a mortgage, making part of it a mortgage-backed overdraft (ANZ call theirs a Flexi loan) is worth looking at. I'm in a similar situation, consulting since 2010. I pay GST and provisional tax every six months. If I've budgeted right, the balance on the mortgage-backed overdraft loan goes to zero right before I send the massive payment to the tax department in May and October.  One problem is that some banks don't like to give these accounts to sole traders. Using a mortgage broker may help get around that restriction.