I went to both islands about 10 years ago in November. It was a bit colder than I expected - I only traveled south of Auckland - if I had to do it again, I would have traveled north to go up to the bay of islands area - would have been a bit warmer too. I don't have any good hiking advice really, as the hiking I did was all on the South Island. That being said, I really wanted to do the Tongariro alpine crossing hike on the north island, but the weather was too shit. Apparently the weather can often be that way, so if you want to do that one, it's best to plan a few days waiting for good weather.