Kayak: Lists all the travel search engines – one stop shop. Also, check on Wednesday mornings, because prices tend to be lower than other times during the week.
Student Universe: With deals saved for students, under 25, and even teachers, this website provided competitive deals and user-friendly customer service.
http://www.intercity.co.nz/: Bus that run routes on both islands (also includes the ferry). If you purchase a pass, you can buy it in increments of hours not fares. Basically, you can take a two-hour bus ride one day and seven-hour the next and only be charged nine hours against the pass! Not two fares like other transit systems. Added bonus, as long as you cancel two hours prior to departure, the fare will not be charged against your account.