I do not work for REI and I 100% agree with the recommendation made by @REI-JenK. I have the Flash 22 (very similar to the Flash 18), and I recently used it as a clothing "stuff sack" in my larger backpack on a mutliday trip. I removed the foamy back pad and it worked perfectly. It's super lightweight, comfortable, has good internal storage options (including a bladder compartment/hook), and is big enough for 5-15 mile day trips from basecamp. And, if all that weren't enough, it's pretty cheap!
Have fun in NZ!