I have created a custom invoice and when i go to create invoice, i select Customise and tick the new invoice style. But the invoice does not change from default?

Hi there, thegumnutproject.

I'm glad to see you posting here in the Community. Let's figure out why you're unable to change the invoice template within the invoice.

I've doubled-checked here and haven't found any reported issue specific to the Australia region. I also tied to replicate the issue and was able to succeed with changing the invoice template.

Let's perform some basic troubleshooting to determine the cause of the problem. Please start by making sure you're using a supported browser for QuickBooks Online.

You can refer to this article for more insights on the supported browsers: 

System requirements for QuickBooks Online, Accountant, and Self-Employed.

If the problem persists, I suggest clearing its cache or switching to a different supported browser. In case you need to contact the QuickBooks Customer Support team, I'm attaching the article I recommend on this:

Contact the QuickBooks Online Customer Support team.

Get back to me if you need more help with changing templates. I'll be sure to lend a hand.