Kiwis are involved in many of the costuming lists online but there are a few local groups that specifically deal with different genres in New Zealand.

http://nzcostumers.forumandco.com/
NZ Costuming, to bring costumers of diverse interests together and share information about events and costuming.

http://community.livejournal.com/nz_costumers/profile
A forum for New Zealand amateur and professional costumers.

Living history: http://www.gatheringdarkness.co.nz/forum/index.php
Gathering Darkness, for a range of living history groups to discuss events and resources.

http://www.sca.org.nz/groups.php
There are several SCA discussion lists in New Zealand for each local group and New Zealand as a whole. The SCA in NZ and Australia has a Tailor's Guild e-list also.

sci-fi, fantasy, cosplay, larp: http://aucklandcosplay.forumotion.com/index.htm
Auckland Cosplay Picnic Forums, includes discussions about other cosplay events around the country. http://outpost42.dr-maul.com/forums/index.php?act=idx
Outpost 42, NZ's 501st Star Wars costuming group.

http://www.rebellegion.com/forum/viewforum.php?f=49
NZ Outpost, Rebel Legion Star Wars costuming group.

http://community.livejournal.com/nzcosplay/profile
There are other costuming communities on livejournal that kiwis are a part of but not targeted directly for them.
http://forums.cosplay.com
There is a growing number of Kiwis utitilising this site.
http://diatribe.co.nz/
DIATRIBE, NZ Live Action Role Playing community

http://community.livejournal.com/gothloli_nz/profile
or http://community.livejournal.com/nz_lolita/profile
Lolita fashions communities for NZ. Not a cosplay group but there is some cross over in discussions.