I just entered it right now and didn't have a problem... so maybe there were a few bugs on the day it went live.

The Ts&Cs are now more clearly set out now that the main page is active.

Chances are, you might be able to get away with just buying a single pack of Doritos and enter 3 times per day, every day, with it... but to be on the safe side, I wouldn't recommend it.

Multiple entries are permitted, subject to (a) only one (1) entry permitted per Eligible Product purchased; (b) a maximum of three (3) entries will be accepted per residential address per day; (c) a maximum of one (1) entry will be accepted per pack purchased (d) each entry must be submitted separately and in accordance with entry requirements; and (e) unrecognised Barcode numbers will be deemed invalid. Any entry that is lodged by automatic, repetitive, robotic, programmed or similar entry methods or agents, including but not limited to the use of a competition entry service, will be void..
Entrants must retain their eligible participating pack(s) (photocopy of promotional pack is not acceptable) (“Promotional Materials”) as proof of purchase for all entries. Failure to provide the Promotional Materials when requested may, in the absolute discretion of the Promoter, render all of an entrant’s entries invalid and forfeit their right to any prize.
So... just in case you win the major prize, or even a daily prize, the promoter may ask for proof of all your entries (in their database under your name), and if you have less packs on hand than entries (in other words, if you used a pack for more than one entry), you are likely to be disqualified.
That last competition from Target didn't ask for proof of purchase with the daily prizes, but you never know.

(besides, I wouldn't be setting a good example if I was encouraging ways to cheat at something)

You do NOT have to provide proof of purchase though... so if you are keen but cheap (or don't like Doritos), find discarded packs in the trash. Since the eligible packs only have to be of the sizes listed on the website, it doesn't look like you need packs with the Transformers competition featured on it.

Eligible Products consist of the Promoters’ products in the varying sizes listed below:
Australia:
• 175g – Doritos Cheese Supreme, Doritos Nacho Cheese, Doritos Original, Doritos Mexicana
• 90g – Doritos Cheese Supreme
• 150g – Doritos Jacked Cheesy Enchilada, Doritos Jacked Buffalo Wings
• 80g – Doritos Jacked Cheesy Enchilada
New Zealand:
• 175g – Doritos Cheese Supreme, Doritos Nacho Cheese, Doritos Original
• 150g – Doritos Jacked Cheesy Enchilada, Doritos Jacked Buffalo Wings
It does NOT ask for "specially marked packs", so since the "Promoter" is The Smith’s Snackfood Company Limited, it shouldn't need to be a Transformers-themed pack... as there is no 3rd party promoter that would require their packs be bought.