Through the end of June, you can snag Windows 10 Professional for US$39.99, an exclusive offer to PCWorld readers.
However when you goto the buy site it's quoted as AU$79.00
Google tells me today that US$39.99 converts to AU$58.56
That seems a solid mark up for converting to AU$ but still way cheaper than buying from MS in OZ.
More about it here
https://www.pcworld.com/article/3562330/exclusive-deal-get-w…
Get it here
https://software.pcworld.com/p36877-windows_10_professional
edit:
I didn't notice this before but you seem to be able to switch currencies back to US$ which may save you money depending on how much you pay for conversion.
I haven't tried the deal as I already use Win 10 Pro. I paid AU retail for it some time back. If you do go for it then please post back how it went.
In for comments on how to get this for free.