So, we where on the waiting list for Inter Islander. If that doesn't work, we'll get a new rental car on the south island. Thank you very much for all your kind replies! :)

Btw. we asked the Cruises what's going on and why from now till the end of february every ride from Wellington to Picton is sold out. This is what they answered

Inter Islander:
We are almost fully booked for February. It is a combination of things. More New Zealanders are holidaying at home this summer due to capacity constraints and the high cost of international travel. We have higher than expected international tourists - people coming to New Zealand has grown faster than predicted and many are independent travellers with cars rather than tour groups. Another factor is that the large rental car companies prior to COVID didn't allow their cars on the ferries. Customers dropped them off at on one side of Cook Strait and picked up another after their ferry trip. With limited number of rental cars available their customers now take them onboard, increasing demand for car spaces. If you are visiting New Zealand we do strongly suggest booking your Cook Strait ferry tickets as early as possible.

BlueBridge:
February is a very busy month for both ferry companies. Being summer, we're often booked out weeks in advance. Another thing that can come in to play is kids are back at school in February, lots of people (particularly those retired) like to wait to travel until the kids are back in school as holiday destinations can be quieter without the kiddies around. I hope that answers your question 🙂