According to the current tax policy of Inland Revenue, gambling is classified as a recreational activity rather than an income-gaining one. It means that, in most cases, Kiwis do not have to pay income tax on their online and offline gambling winnings.
The sole exception to this is professional gambling. If most of your yearly revenue comes from gaming activities, then Inland Revenue might label you a professional gambler. In this case, you must pay income tax on your winnings.