Do yourselves a favour, get yourselfs an IRD (tax) number as soon as possible, and register it with your bank.

Interest earned on your bank account attracts government tax at the same rate as the pay scale, so you will be charged the maximum tax rate on interest (33%) until you register your IRD number.

This will only be applicable to people earning under $38,000 as this is when tax goes from 19.5% to 33%!