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The limit is 2.25 Litres, anything over the limit and you are liable to pay excise/duty/tax on the full amount, not just that over the limit. However, with beer it may not be that much.
Here are the current rates:
Schedule - (1 July 2010)
On top of that there's also 10% GST payable; both on the excise and on the purchase price. Lets assume a six-pack has 2 litres of beer (6x333ml) at 4.5% alcohol by volume.
If this 2 litres of beer was all you had no duty or excise would be payable.
If you had 3 such six packs (6 litres of beer) the excise would be calculated according to this:
Beer exceeding 3.5% by volume of alcohol packaged in an individual container not exceeding 48 litres:
$41.68 per litre of alcohol calculated on that alcohol content by which the percentage by volume of alcohol of the goods exceeds 1.15
The first 1.15% of Alcohol per litre is not charged for, but the amount over that (3.35%=4.50%-1.15%) is.
So 6 litres of this beer has 6 x 3.35% or 0.201 Liters of excisable alcohol, so that cost is $41.86 x .201 or $8.41.
Now GST is payable on that, so add 84¢ to get $9.26.
We have not finished yet. GST is also payable on the purchase price. Lets say the special beer you are importing cost USD8.50 per six pack which may be considered worth AUD10.
So another $3 in GST on that $30 must be paid giving a total of $12.26.
Seriously, if you declare such a quantity of beer, they may just waive you though not wanting to go to the hassle of calculating the excise/tax for such a paltry amount. Then again, it may be considered good practice for someone.