Our travel from the South Island to the North Island started in Picton and ended in Wellington. We booked our ticket in the Motueka i-SITE Visitor Information Centre. When planning the ferry travel, plan in a whole day.
The ferry ride takes only four hours, but you have to be there at least one hour before the ferry starts. As boarding, parking and filling the boat will take a while. Leaving the ferry took us also nearly one hour.
I would recommend taking the early ferry in order to be able to find a camping side in Wellington or around. We actually struggled with that and ended up in huge expensive camping side in Wellington.
One nice tip for the travel. In case you want to save some money, the ferry crossing is as beautiful as booking a Fjordland boat ride. Just go on deck and enjoy the view. In case there is rain or it is too cold, it is also possible to watch the surrounding inside, through the many windows. Also, if you are flexible with the date you want to take the ferry, there are often special offers on specific days of the week when you can get the tickets for the half price.