One thing to be aware of is that in July for the South Island, weather could potentially have a larger influence that during the Summer or Autumn. I love the South Island, but there are mountain passes that do regularly get closed during the winter months. Depending on the route you take in the South Island there could be three or four alpine passes. The good thing is they are normally only closed overnight or for part of a day, but it does. have the potential to slow things down. Make sure if you opt for the South Island your schedule isn't incredibly tight.