Hi Steve,
The weather in the autumn (March through to May) is very pleasant throughout New Zealand and certainly a little nicer than the British weather. You will be looking at temperatures between 20-10 degrees and maybe a little cooler at night.
For most of the tour we recommend comfortable lightweight clothing. However, it is essential to take a waterproof jacket and warm clothing for the cool evenings and more exposed locations.
Kindest regards,
Monty