November 21 is the actual date that Steam dumps US pricing for Australian users and switches to our native currency, however, it appears Valve has already pulled the trigger on publishing an info page, which contains a short FAQ on how things will change.
You'll find the announcement — which notably uses the present-tense — on the Steam website. If you're unable to see it, here's the text in full:
The entire Steam catalog is now priced in Australian Dollars for customers living in Australia.
Frequently Asked Questions
What happened to USD in my Steam Wallet?
Australian accounts with Steam Wallets in USD have automatically been converted to Australian Dollars, at a conversion rate dictated by market value at the time of conversion. To view your wallet balance and account details, go to https://store.steampowered.com/account.
Can I continue to make purchases in USD?
The Australian Dollar is the currency that is available for purchases made in Australia.
Has the availability of games on Steam changed with this?No changes have been made to the contents of your library. All of the languages and features in the games that existed when the store was priced in USD are still available. Most games previously available in USD are now priced in AUD and are available for purchase in Australia, but an occasional product may be missing until it is priced in AUD.
Despite stating the price change is in effect, a quick look at the store via the Steam client will show this isn't the case.
In addition, Australian users have begun receiving the following email:
Steam will soon be priced in Australian Dollars!
If your Steam wallet has any money in it on November 21st, 2018, your U.S. Dollar balance will be converted to AUD, at a conversion rate dictated by market value at the time of conversion.
To view your account balance, simply log into your Steam client or the Steam website at http://www.steampowered.com. Once you have logged in, you will see your balance in the upper-right corner next to your account name.
Sure, it's a little confusing that the announcement page is live on Steam, but from what I can tell, there's no physical link to it. That said, it can be found rather easily with a Google Search.
My guess would be Valve is just getting things ready for next week. Either that, or someone pressed the publish button by mistake.
Years after integrating support in the developer backend for Australia as a currency and a region, Steam has revealed to developers that customer-facing support for Australian dollars will arrive mid next-month.