A voluntary agreement that you make with your ex-partner is a written child support agreement that's registered with Inland Revenue. You and your ex-partner decide the amount, but Inland Revenue takes care of collecting and paying the money, as well as following up on missed payments.
If at any point you need to change a voluntary agreement, you do this through Inland Revenue. This includes letting Inland Revenue know if:
Find out more about voluntary agreements on the Inland Revenue website(external link)