Still another month left on this competition, and after a couple weeks of full-price Doritos at both major grocery stores, they are on sale this week at both... for anyone still wanting to submit entries for that grand prize (or the daily prize of the cooler).

Coles - half price at $1.60 each.
Woolies - $2.50 (save about a $1).

Your odds might be better for the daily prize at this back end of the competition, if people have lost interest in entering after the hype of the movie ended.
(also, since the wording of the competition doesn't say that you need marked packs, if the stores start stocking regular packs soon, they should be still valid)