A GST invoice template that does the maths

Inclusive or exclusive: the part templates get wrong

A downloadable Word or Excel invoice template has one job it cannot do — arithmetic you can trust. The most common error on small business invoices is mixing up GST-inclusive and GST-exclusive pricing, and it is an easy mistake because the two look identical on the page until someone checks the total.

If you quoted a client $1,150 including GST, the GST component is not 15% of $1,150. It is $150 — because the GST-exclusive figure was $1,000. Working backwards from an inclusive price means dividing by 1.15, not multiplying by 0.15, and getting it wrong overstates the GST by more than 17%. Do that on a $20,000 job and it is a real problem, not a rounding error.

This template asks you one question — are your prices tax exclusive or tax inclusive — and then does the calculation correctly in both directions, always showing the GST component as a separate line, which is what a compliant tax invoice requires. Switch the rate to 10% for Australia, 20% for UK VAT, or type any custom rate.

What a compliant tax invoice has to contain

An invoice is a request for payment. A tax invoice is also the document your client uses to claim back the GST they paid you — which is why tax authorities are specific about what has to appear on it, and why a missing field can hold up your money while an accounts department sends it back.

In New Zealand the rules were modernised in 2023 and the old requirement to hold a formal "tax invoice" was replaced by taxable supply information. The practical checklist did not really change:

• The words Tax invoice in a prominent place, or a clear indication that the document is one.
• Your registered business name and GST number.
• The date the invoice was issued.
• A description of the goods or services supplied, with quantities where relevant.
• The amount payable and the GST charged — either shown separately, or the GST-inclusive total with a statement that GST is included.
• For supplies over $1,000, the buyer's name and address as well.

If you are not GST registered

Do not charge GST, do not show a GST line, and do not call the document a tax invoice. Set the tax mode to No tax and the heading changes to "Invoice" automatically. Charging GST when you are not registered is a genuine problem, not a technicality — you would be collecting tax you have no right to collect.

Registration becomes compulsory in New Zealand once your turnover passes $60,000 in any twelve-month period, looking both backwards and forwards. Below that it is a choice, and it is not automatically the right one: registering means charging your customers 15% more and filing returns, which is worth it if your clients are businesses claiming it back, and often is not if they are consumers.

Invoice numbering, dates and getting paid faster

Your invoice numbers need to be unique and sequential. That is the whole rule. INV-0001 upwards is perfectly good; adding the year (2026-014) makes end-of-year filing easier; per-client prefixes (ACME-007) help if you invoice a handful of large accounts. What you must never do is reuse a number or leave gaps you cannot explain — both make a GST audit slower than it needs to be.

Payment terms are where most small businesses quietly lose money. Three things move the needle more than chasing:

1. Invoice the day the work finishes, not at the end of the month. Payment clocks start when the invoice arrives, so a week of delay on your side is a week added to your own wait.
2. Put the due date on the invoice as a date, not as "14 days". "Due 12 August 2026" is unambiguous; "payment terms: 14 days" invites a debate about when the clock started.
3. Make paying trivial. Bank account number, and a reference that matches the invoice number, on the invoice itself. Every extra step is another day.

If an invoice goes past due, a short factual email on day one beats a stern one on day thirty. Most late payment is administrative, not hostile — the invoice went to the wrong inbox, or it missed a payment run.