In the Shopify app store are several loyalty plugins
do some research for the one which will best suit you.
I did a search for “Loyalty rewards”

https://apps.shopify.com/search?q=loyalty+rewards

there are plenty for you to choose from.

Also go back to your developer who built the shop for you – they may have some recommendations. There are lots of good Shopify agencies in New Zealand – search for them and ask their advice.

Rebecca