A great way to use your Qantas Wine Voucher is to pair it with a bonus points offer.
Remember, you also get free delivery if you're a Qantas "Points Club" member and triple points on cash spent.
If you buy one bottle using the Points Club voucher, you get a bottle of wine and 1000 Qantas points for free (infinity points per dollar?).
If you buy two bottles for $70 and use the $50 Qantas Points club voucher, it works out at $20 for 2060 points (103 points per dollar).
I'm not going to argue that 0.97c/point is bad value, but personally, I think that it's a bit of a waste as 2060 points isn't going to get you far. A better deal right now is something more like the Christmas Collection Travel Bonus Dozen @ $498. If you're Points Club, then that will net you 25494 points (at 1.95c/point) which is enough for 1.5 return SYD>MEL flights. If you have a $50 voucher to throw at it, then you'll still earn 25344 points (at 1.76c/point).
Even better, if you're PC+ then it's $448.20 for 25345 points (1.76c/point), and if you have a $50 voucher, it's $398.20 for 25195 points (1.58c/point).
Sure, $20 < $398.20, but I guess YMMV in terms of what you see as "value" from Qantas Wines.