How do you calculate $20 more for a carton?
The excise is going up as it's indexed to inflation.
The rate is applied to alcohol content. i.e a tax rate per litre of alcohol which varies depending on the size of container it comes in and the beverage.
https://www.ato.gov.au/businesses-and-organisations/gst-excise-and-indirect-taxes/excise-on-alcohol/excise-duty-rates-for-alcohol
According to the Brewers Association consumers will be paying nearly $20 total in tax for a slab after the latest rise not $20 extra in tax.
