LARP is sold by word of mouth, so the best way to build a larp community is to run games. Pick something with a bit of flexibility (e.g. Much Ado About Something, stick up some ads, and see who bites. Set up a Google group at the same time for local notifications and get people talking. Hopefully it won't take long before they're talking about running stuff. And obviously, point people here and at NZRaG so they can see what else is out there.
Games with low costume requirements
* any of the modern Shifting Forest stuff (all 8 players, unfortunately)
* Battlestar Galactica: The Black Corridor (BSG civillian clothes are just ours, twisted a bit; can be run in a flat)
* But Nobody Loses an Eye (I'm sure people can do kids)
* Down and Out in Middenburg (fantasy beggars, so wear a sack or old SCA clothing)
* Prayers on a Porcelain Altar (students)
* A Serpent of Ash (modern people)
I'd initially aim for a game a term, rather than one a month, to prevent burnout.
Regular games and getting people talking should hopefully start a nice positive feedback cycle and help build things even more.