As the others have said, do it in two days with a stop at either the Cavallis or just inside the Whangaroa entrance. Through the Cavalli Passage (ie inside the islands) is shorter and more scenic - the way through is relatively easy if you know how to read a chart or use a GPS. As you head further down on the second day, don't stick too close to shore as there are some rocks off the northern side of the Purerua Peninsula (in saying that you would need to be quite close in for them to give you any problems). There's plenty of water inside the ninepin rather than having to go around the outside.
