You have explained the withholding tax, there are also people who don't pay withholding tax as they are regular employees who have a tax number, they need to pay income tax which is being withheld on a monthly basis. After the tax year ends the employee needs to fill in form where they state their income, tax and social security paid and any deductions they think they have. Then the tax office will do an audit and declares what the definite tax for last years is. People then either get money back or have to pay additional tax.