September is normally spring skiing, but last season it had the colder conditions we normally get in August, and August was spring skiing. The wacky weather patterns have been playing havoc, looks like North America is no different this season.
Treble Cone closes at end of September/first week of October, even if it still has great snow. Crowds drop off then and its not worth staying open. The Queenstown resorts and Cardrona stay open trhough Octiber until the snow is gone. The school holidays are usually the last week of September, so the start would be better.
I also think September is the most beautiful month of the whole year to visit the South Island - scenery is awesome. It is starting to warm up, still lots of snow on the moutnains, lakes are really blue from the cold snow feeding them.
I don't know what dollars you are bringing but they will go a long way. A lift pass at TC (single day) was $58 NZ last season. A US dollar is less than half that! Dollar for dollar meals and equipment are similar in prices, so everything will seem real cheap! This includes ski gear, I bought Rossi bandits in 1999 for $899 NZ, went to Canada at start of 2000 and they were $840 CAD. We only got 60c Canadian for our NZ dollar. Hence we bought little gear on our holiday. Mountain Hardwear clothing is cheaper here too - an Etheral FTX Jacket was $800 NZ last season, we saw them for $700 - $800 CAD over there. Don't know why, seems crazy. A ski shop owner we know in Queenstown regulalry has merican families coming in and outfitting themselves from head to toe, cause it is cheaper than at home.