Just cancelled my sub and then repurchased. Pricing is correctly displayed.
Double check your invoice/receipt? - You should not have been charged GST on the gift card. I sell gift cards in my company, and no GST is chargeable on gift cards themselves. GST is charged at the time of purchasing the items that you pay for your gift cards. The gift cards are, in effect, cash. If you had a 19.99 gift card for Zwift, eg, you’d have $19.99 to redeem. The monthly cost is AUD$19.99 PLUS GST - so the cost is AUD21.99. Your card is $2 short - hence the extras.
Has anyone received an email about the change? If so please can they post it. I’ve searched my notifications from Zwift and can’t find anything from July 2019 about being able to pay in AUD. I’m paying US$16.49 which is AUD $24.81 via Paypal. How long does it take for the membership to be cancelled - does it run to the end of the payment period or is it effective immediately?
The email explaining the addition of taxes to your subscription should have been sent out several days before the first charge that would have been affected. More information on the change can be found here.
As for the option to pay through non-US currency, that was added a while before then. The system wasn’t able to automatically switch you over, but canceling and starting a new subscription should fix it. In case you need instructions for canceling, that would be here.
Additionally, your membership will continue until the next bill date regardless of when you cancel.
i got a bit sick of this overcharging as well. i got a one year sub from wiggle - they charged what they said they would and the physical card arrived in the post.
they are out of stock at the moment, and they sell out fast, so should tell you? 
zwift, its pretty simple, if you are charging $AUD 21.99 then put that in your pricing cost, not $19.99 as you do. Residents outside of the USA don’t put up with the ridiculous price creep that occurs in your country. oh, that has tips and state tax extra…
In the Czech Republic, EUR and USD are not the national currency. I have to be given a choice
If you use one of the accepted methods of payments (named credit cards, paypal etc), they will do the currency conversion for you. Does this not work?
I have membership more then 3 years. Since begging till August 2020 I was charged USD 14.99. Then without any notification the charge has been changed = increased for EUR 14.99. Zwift mention that charge should be equivalent of $14.99 in local currency. It means it should be circa EUR 12.45, not EUR 14.99!!! I don’t understand how it can be increased without prior approval by customer…? It could be system error but in such a case the difference should be reimbursed, shouldn’t be? Response from Zwift support is no, because of automated system. Very strange…