New tax code won't work

I'm using the US version but things should work in a similar fashion....

Creating a new sales tax item is the recommended method here in the US. That keeps old and new taxes correct in case you ever need to go back and revise an invoice prior to the increase.

Are you using sales groups? If you are then you need to create a new sales group that uses the new sales tax Item.

I would also double check those invoices that show the old sales tax rate and confirm that the correct sales tax item was selected on the invoice.

A sales tax group is a special Item that consists of multiple sales tax codes. A typical use would be if you had a county, city and state taxes and each location had different rates. The sales groups would combine the various sales tax items to be used on the invoices.

When you create the invoice are you selecting the correct sales tax item at the bottom of the invoice screen or when you created the service item?

The NZ setup is different than how we setup things in the US. We don't have the option of associating a sales tax item with a item. We use the sales tax item on the bottom of invoice or sales receipt only. Do you have that option? One advantage of this method is that you don't need to create new service items for each tax rate. you just create the new sales tax rate and apply it where needed.