Child Support Act 1991, ss 35(3)-(4), 40AA-45
For child support purposes your taxable income is based on the previous January-December calendar year if you’re a wage or salary earner, and in other cases on the basis of the tax year (April to March) before the last tax year. However, if you expect your current income to be at least 15% less than your income from that earlier calendar or tax year, you can apply to IRD to allow you to estimate your income for the current year, that is, what you believe your income is going to be for the year.
If your income changes more than once in a year, you can apply again for a different estimate to be used. However, IRD won’t let you do this if you’ve already applied within the last three months and if your proposed new estimated annual amount would change the existing estimate by only $500 or less.
Note: Under the child support laws which came into force from April 2015, both parents’ incomes are relevant to whether child support will have to be paid and how much. Either parent, not just the liable parent, can now ask Inland Revenue for their income to be estimated.