December to February is summer in New Zealand—and the height of tourist season. Which makes your question of whether February is a ‘good’ time to visit moot. Prices will be high, the roads full (by New Zealand standards!) but the weather will be perfect and all activities on tap. Personally, I would travel a few months later but then again, you may not want to postpone a honeymoon.
Your next question, regarding operators, is a more vexed one. This basically comes down to personal preference, the amount of time available and motivation levels. As a rule, I prefer independent travel; however, I do appreciate the utility of a travel operator-organised trip. But it’s a qualified approval: it’s specifically useful when a) you’re travelling to non-English-speaking lands, or b) places where tourist infrastructure and information is limited and c) you have limited time for trip planning. Both a and b are not relevant in the case of New Zealand (and c is something you can best estimate yourself). I suggest you first take a look at newzealand.com, the official website for New Zealand Tourism. It’s very well-organised, giving you all possible trip-planning information, right down to detailed itineraries for different holiday lengths, with links to accommodation, transport options, things to see and do, driving times—the works. If you have the time and energy, you can easily build your holiday using just this website. Fun, too!
But do also take a look at what’s on offer at major tour operators such as Cox & Kings (coxandkings.com) or Thomas Cook (thomascook.in). Both offer New Zealand itineraries that include self-drive segments. Thomas Cook, in fact, has a New Zealand Honeymoon Package (15N/16D), which takes in Auckland, Coromandel, Rotorua, Queenstown, Wanaka, Mount Cook and Christchurch. It’s a packed trip, which has the virtue of making sure you see pretty much all the highlights. On the other hand, it’s expensive—prices per person start at ?2,36,000, not including airfare and several meals. A happy compromise might be to enquire about a customised holiday.