You could try a google search of yacht charters no-one else will likely know availability.
I would think your chances are slim. There would be few boats under 30' though there was one years ago. Second summer particularly the Christmas period is peak holiday season, so most would be booked out months ago, probably with a minimum one week.
A private charter might be available while strictly illegal, but most owners would be reluctant for insurance reasons, risk, etc. At a minimum they would want proof of experience. I doubt any would bother for a few days.
On recent months you could expect a high chance of 20 -30 knot winds though 2-3 days a week may be 5-15.
On a local level you might try crew.org.nz . Otherwise you may find your best chance is getting a day sail.