TLDR: January is the busiest time for travel between the North and South Islands.But, if you are planning to cross mid/late January without a vehicle you should be fine to wait until you are in the country.
January is probably the busiest time for travel between the North and South Islands. However, the difficulty in booking depends entirely on when you might be aiming to cross. If you are flexible and planning to cross mid/late January you should be fine to delay until you know your plann. Here are some things to think about...
First - will be you be travelling with a car / camper? If so, try to arrange to drop it off when you cross and pick up another on the other island. The ferry is far more expensive with a car / van, and you are more likely to need to book in advance.
Second - the busiest time over summer is Christmas to New Years and then the few days in early Jan before everyone goes back to work. Judging by public holidays, many people will return to work on the Mon 7th next year (2019), so expect it to be busy before that. It will quieten down after that but weekends may still be busy. Travelling mid week should be fine to book on a few days in advance.
Also, most schools will start the year from the 28th to the 7th Feb so there will be another busier period then.
Third - your options to travel. There are 2 ferries that cross between Picton and Wellington:
There are also flights between Wellington and Blenheim, Picton, Or Nelson through Sounds Air. Plus Air New Zealand and Jetstar between the bigger cities. So you have plenty of options when it comes to travel and should be able to get something at shortish notice.
Sounds air and the ferries are both great trips on a good day.