A tax invoice is an invoice issued by a business registered for GST that contains the details the ATO requires. A customer who is also registered for GST needs one to claim back the GST they paid you. Only GST-registered businesses issue tax invoices; a business that is not registered issues an ordinary invoice and must not show GST on it.
The ATO's requirements for a tax invoice are that it shows it is intended to be a tax invoice, your identity and ABN, the date it was issued, a brief description of what was sold including the quantity and price, the GST amount payable, if any, and the extent to which each item includes GST. For sales of $1,000 or more, it must also show the buyer's identity or ABN. A registered business must provide a tax invoice within 28 days if the customer asks for one, for taxable sales over $82.50 including GST. Check the ATO's tax invoice page if your template is missing anything.
In practice
Homeowners cannot claim GST back, so they rarely care whether an invoice is a tax invoice. Builders, property managers and commercial clients do, and their accounts teams will return an invoice that is missing the words "tax invoice", your ABN or the GST amount.
A plasterer invoices a builder $3,300 including GST for sheeting an extension. The tax invoice shows "Tax invoice" at the top, the plasterer's business name and ABN, the date, the builder's business name and ABN because the sale is over $1,000, the description "Supply, hang and set plasterboard to extension, 64 square metres", the total of $3,300 and the GST included, $300. If the builder raised a purchase order, its number goes on too.
The simplest way to stay right is to build these fields into your invoice template once and to make each invoice from the accepted quote, line for line. Then the description and quantities already match what the customer agreed to, and the only thing to add is anything that changed through an accepted variation.