Depends on what sort of alcohol.
Beer is not expensive. You can even buy them via vending machines and most convenience stores now (ie 7-11, etc).
Again, depending on the brand, you can probably get a 350ml can for about 3AUD. For some reason, beer are mostly sold in cans than in glass bottles.
Wine, hard liquor, etc *might* be a bit more expensive compared to Australia.
Just be aware though, I do not believe one can buy any duty free on arrival at any major international airports in Japan. I think your last chance will be on the plane.
If you are a visitor, the official alcohol allowance is 3 bottles of alcohol (roughly 760ml each). Don't ask me why 760ml bottles and not 1L, etc. I rarely buy alcohol when departing via BNE Int'l. But my impression is that the bottles are usually at least 1L or small enough to hold in your palms.
Enjoy your trip!