When we went about 3years ago it was in Feb & it was a last minute decision to go as we found out a long time family friend was sick. I had just started a new job & wasn't too keen about approaching the boss about it, anywho Dad rang me each day for an answer as with in a week flights had gone up $200!! So if you see a good deal or something you can afford I would book it.
As someone at Qantas exp to me once, they have certain amount of seats for each price range, you can see the different price range & whats available online. The flights themselves dont go up in price, the cheaper seats get filled first & as they get fuller the next lot of seats get more exp...IYKWIM. So you should be able to go online & see what looks available for that time & if the cheaper one's are available I would book them now, as apart from some great offer coming up...you run the risk a heap of people book the only cheaper seats before you book yours & you get stuck having to pay more. HTH.