You can find out more on the IRD website.
Resident Withholding Tax (RWT) is a tax deducted from the gross amount of interest paid to you. It is deducted at the time when the interest is paid to your InvestNow cash account.
When you open an InvestNow account, you will be asked to select your RWT rate and IRD number.
If we do not have an IRD number your account will be taxed at the maximum “non-declaration” rate of 33%.