There are a variety of airlines that fly here (NZ) with differing prices/quality/service/ number of stop overs etc. so the best choice depends on your requirements.
Once you are here it is really easy to get around and find places to stay so there isn't always a need to use a company to book your trip for you. Travel Agents will be able to help prebook things or if you have the time some internet searching will help.
If you have any specific questions about a trip here let me know I'll try and help.