Tax (VAT) not calculated correctly at checkout

[weatherspares]

Good morning.

I'm a fairly new Shopify store based in the United Kingdom with 20% good tax (VAT).

All my products are set to include VAT in the store at 20%. A UK customer places and order and the checkout system correctly operates. A customer buys a £60 product and £10 is calculated as VAT and shown on the customer order page.

Today my first New Zealand customer placed an order for a product costing £59 including VAT at 20%.  The customer should of had 20% VAT removed from the price reducing this to £49.17, however, although the customer record shows that VAT was 0% and £0 was charged the amount was not reduced and the customer was charged the full amount.

A screenshot of the customers order is shown (after I manually refunded the VAT)

Any help would be much appreciated.