This week at Woolies, Doritos are less than half price, at $1.50 each... so a good time to buy up a few packs to put away (the current batch don't "expire" until November) and enter the competition.
Just keep it to 3 per day, and one entry per pack... and keep all your packs until the competition ends, in case you win the big prize and they need proof later of all your entries.