August is the most reliable month, September if you like spring skiing.
Web site of all resorts www.snow.co.nz
New Zealand has quite a variety of ski areas. There are commercial areas and club areas. Club areas are managed by members/skiers, not a commercial area.
Treble Cone, Wanaka - if you like a challenge but want a chairlift to sit on, and an expresso at lunch time. My favourite commercial area, never get bored here. Awesome views, located in most beautiful part of NZ.www.treblecone.co.nz
Craigieburn, - club ski area with rope tows. Challenging slopes, no groomers, no crowds (I've had epic powder days with only 20 kiers on the mountain), no facilities except lunch hut with awesome views and sun deck - bring your own food, buy a cup of instant coffee or take your own thermos. Can buy a ski week here with on mountain accomodation, food, tows and instruction for about $552 NZ a week (this is $250 USD - very cheap). Not recommended for snowboarders (difficult to ride rope tows) or beginners/intermediates. Glen Plakes favourite area in New Zealand.http://www.craigieburn.co.nz/