I've seen a lot of posts here asking about invoicing software when starting out as a freelancer or sole trader in NZ. Most people end up on Xero or MYOB and pay $50–80/month when they don't need to.
Here's what IRD actually requires on a tax invoice:
The words "Tax Invoice" at the top
Your GST registration number
Your name and address
The date
A description of the goods or services
The GST amount (15%) shown separately
The total including GST
For invoices over $1,000 you also need your client's name and address.
That's it. You can do this in Word, Google Docs, or any basic invoicing tool. You don't need a $60/month accounting suite unless you're doing full bookkeeping.
Happy to answer any questions about GST invoicing — it catches a lot of people out when they first start freelancing.