Whats the GST? New Zealand, adds and takes away 15% Goods and Services Tax. Use the Add 15% GST Calculator to work out what the pricing is after-GST has been added, an equation formula will be shown which reveals the value of the tax and the final price. Or alternatively use the Minus 15% GST Calculator to work out what the value of the product was before-GST was added. Custom after-tax and before-tax calculators included. Perfect for start-ups and businesses!