Because someone asked me REALLY nicely, I've duplicated the Australian GST calculator and modified for New Zealand (15% GST).
The free online New Zealand GST and New Zealand GST Free Portion Calculator! Provided here for your use. Please make sure you report any issues with the calculator in the comments below. Enjoy your calculations!
Take note, this is the New Zealand GST Calculator, which is 15%. Please ensure you let me know if there is any issues with it's calculations in the comments below.
Please feel free to use this script as your require, if you wish to have this script on your own website please post an acknowledgement and link back to this page.