Your data allowance renews on the first of each month. If you're on an Unlimited plan then there is no renewal date.
You are sent your anniversary date via email when you first sign up, otherwise you can check it out in My One NZ
.
Simply click on the "Check Usage" link from the left-hand menu.
Here you'll be able to see your current and past data usage, the dates listed in the drop down list show when your monthly data allocation starts and ends.