Refer this link:

http://www.ird.govt.nz/gst/gst-registering/gst-about/

Goods and services tax (GST) is a tax on most goods and services in New Zealand, most imported goods, and certain imported services. GST is added to the price of taxable goods and services at a rate of 15%.

Here's how to calculate your GST:
• You charge GST at 15% for sales and income. ◦ Calculate GST by multiplying the sales and the income figure by 3 then dividing by 23.

• You claim GST at 15% for purchases and expenses. ◦ Calculate GST by multiplying the purchases and expenses figure by 3 then dividing by 23.
Note:

Multiplying a sales or purchases amount by 3 then dividing by 23 enables you to find out the GST amount. Subtract this amount to get the GST exclusive amount.

3/23 = 0.1304348 = 13.04348%

So is the GST rate 13.04348% or exact 15%?

GST rate is 15%

So $100 + GST is $115

To find the GST content of the total $115 x 3 = $345 then / 23 = $15

So the GST content of the total is $15.

Very good, did you find that on facebook or twitter ??

How about just multiplying by 1.15? Seems a lot simpler to me...

Oh dear, the IRD are gonna love you.

How does your simple theory work if something costs you $180? How much do you claim as GST

Hi
If $180 includes gst at 15% then you can divide $180 by 115 and multiply by 100, this gives the net goods figure $157.
$180 - $157 = $23 ( this is the GST charged on the net goods )
Trust this helps
Ian

Then you simply divide 1.15 and the difference between the two is the GST amount.

It's not difficult!

Multiplying the price by 3/23 tells you the amount of GST you have paid. It's the fraction of the total price that is GST.

Geek, the amount you multiply by 3/23 already includes GST