Aussie stores may be seeing a notification message in their Admin dashboard that looks like the image below. 

In case the image doesn't load it reads:

GST will be applied to your next Shopify bill.
If you’re registered for GST, add your Australian Business Number (ABN) before your next bill to stay tax-exempt.

I've seen some people get confused by this message.

It doesn't mean that throwing in any old ABN will make you tax exempt. Shopify - having an Australian entity - needs to charge GST and you still need to pay tax. 

If you have a Tax Exempt ABN, adding that in will make sure you don't get charged the Australian GST. That's what the message is referring to.