Wellington's official call for Elven extras has appeared on TORn and TradeMe (NZ's auction and jobsearch site).
Applicants have to be between 17 and 40 years old, and women have to be at least 175cm tall (5'9"), while men have to be slightly taller at 183cm (6').
To apply, just turn up to the address listed, and get in line - I'm sure most of Wellington will be there (at least those tall enough).
Alternatively, if you can't make it there on the day - there's an "Apply Online" button on the TradeMe site, they will send you an application form instead. Please though - they will have LOADS of applications to deal with - only apply if you actually pass the height test! There will be other calls later for different races (I'm hoping for open calls for Men and Dwarves later).
Note also, that this is just the Wellington call - there may be others later in NZ cities closer to you; at least if you're in New Zealand. If you're not - perhaps that working holiday visa is becoming a good idea? (NB - having a visa is not a guarantee of getting into The hobbit, obviously! But NOT having a visa is a guarantee of NOT getting into the movie).
On a personal note - annoyingly, my son can apply although I'm too short. I expect that it will be years before he'll let me forget that. Maybe I'll get in as an Orc or something later, and we'll fight on opposite sides of the Battle of Five Armies. Let's see his pointy little ears then! Kids! Pah!