Uhh you night want to consider chartering. That's like 3 full bus loads. It'll be a mess if you try to book 150 people from Christchurch to Queenstown on regular services. People will be leaving at different times etc. It could also end up costing a similar amount. Better to pay more but travel as a group.
Are you flying out of Queenstown? If so, why not fly into Queenstown too and save yourself 6 hours of sitting on a bus. If no flights are available into Queenstown it could be worth talking to airlines and seeing if you can do a special flight or possibly charter a flight direct to Queenstown. If they know they're going to get a block of 150 seats they might consider it.
Bus companies around christchurch:
http://yellow.co.nz/search/christchurch/Bus+Charters&Services-1.html