Yes it is a bit late to find availability for Dec/Jan now... but... that aside, no matter whn you look to book there are potentially two issues ... one is the season... people returning home after Christmas break and students coming to Asutralia to start or return to studies. There's plenty of money out there to pay business class. The second is Air China availability itself... Lifemiles doesn't always have Air China available, and sometimes even if they do, you can't actually book it (you get an error at the page to enter passenger details, or to complete payment).
I would not be making a purchase of Lifemiles solely in the hope of travel on Air China.
You could potentially find availability on Thai via BKK, or Asiana via ICN. But those connection might not always show on LM and might require a screenshot booking (you take screenshots of availability on individual legs and send those off to Lifemiles to manually ticket them).