Duty free in Australia, at least for spirits, is (almost) always going to be cheaper than the normal retail prices - simply due to the relatively high taxes Australia has on spirits. That's very different to the US where the taxes are lower, and duty free is almost always a bad deal compared to somewhere like BevMo or even the local supermarket!

The issue is of course competition - retail has it (to varying extents, depending on where you are) which helps bring the prices down, duty free doesn't which keeps that up to near full price, just without tax.

The prices SpannerSpinner has shown are fairly typical (I see $70 on the First Duty Free website, and $82 at Dan Murphys, but close enough) - but if you're staying in (for example) the Sydney CBD then you might find it's somewhat more expensive than that - there are no Dan Murphy's in the CBD, and for the other chains that do exist (eg, Liquorland/Coles Express) it's not uncommon for their discount prices to not apply at the CBD/"Express" locations.

At the end of the day it mainly comes down to convenience - If you want cheapest, check the prices online and buy at either Auckland or Sydney duty free. If you couldn't be bothered carrying the extra weight just to save a few dollars, by locally when you get to Sydney.