You get two return trips to NZ paid for by CTC as part of the course - out to NZ, back to the UK for the first bit of leave and your first bunch of ATPL exams, then out to NZ again and back after the end of the NZ multi-engine phase.
There is the possibility to get back over the Christmas break if you so wish, but that does come out of your own pocket and it is expensive at that time of year - and the break isn't really that long. Some (like me) thought it was well worth it, others take the opportunity to explore NZ while the weather is good or just hang around Clearways and enjoy the sunny Christmas atmosphere! You don't get any other holiday as part of the course other than the specified leave periods back in the UK (i.e. there are no weeks off as part of the course when in NZ.) If exceptional circumstances do arise you can request leave, but you must be able to justify it and it mustn't have an adverse impact on your training - don't automatically assume you'll be permitted to have time off.