I'm a Japanese working in Tokyo for over a decade.
Let me answer to your question.
Firstly, let me comment on "buy a data SIM card in Australia for $59Aud valid over 30 days with 7GB."
When you look at its price and conditions alone, I'm sure to say that is reasonable.
However, when you use it, it might be good if you take about connectivities like: Can that Australian SIM card can get frequencies in Japan?
You can get more knowledge from articles like this.
Useful Knowledge to Use Internet in Japan: 5 Networks of Pocket WiFi / Prepaid SIM Card https://pocketwifi.me/wifi/366
Other than that, as Frankie said to you, you can buy a prepaid SIM card in lots of airports in Japan, including Fukuoka airport.
One thing I would like to add that.
Although it depends, you might able to get more benefits if you rent a SIM card rather than buying a prepaid SIM card. It's more reasonable, reliable, and refundable.
A brief comparison can read at:
Differences Between Prepaid SIM Card and Rental SIM Cardhttps://pocketwifi.me/sim/790
Hope you'll get a wonderful experience in Japan.