This one I stumbled across purely by accident while booking tickets on the Event theatre website... but looks like this one is only open to existing Optus (phone or broadband) customers. So unless you are a customer, know someone who is, or plan to be a customer before this ends on 11th July, you might as well skip reading the rest.

Optus customers who have registered to the Optus Movie Rewards site can enter a competition to win a major prize of a trip for 2 to Melbourne for 3 nights in a 4-star hotel, to do a couple of 'extreme' activities (a monster truck ride and a flight simulator), plus $250 spending money... worth $7000.
A second prize is for 11 tickets to a private screening of DotM on 18th July at either a location in Sydney or Melbourne (requires you to make your own transportation if not living there, otherwise the winner can get 11 double-passes). Prize also includes each person getting a pair of TFs headphones and webcam. Total value $2,710 (Private Screening for 11 people valued at $1500, Webcams - $45 each, Headphones - $65 each)
Then there are 20 runner up prizes of a double-pass to DotM, valued at $50 each.

Conditions listed here.

It mentions that registering to the site earns you (free) entry, but also says that existing members can 'purchase' an entry by buying certain TFs items they have available (music, games, ringtones, pics, movies).

It also appears that existing customers may have even earnt free tickets to a screening of DotM on the 30th June (day after it starts).