After trying some USB chargers I bought in NZ, and not having much success (very little in the way of large capacity ones, often overrated etc..) I did a google search and found the LimeFuel ones to come highly recommended. After a bit of trouble getting one here as you can't ship them to NZ from Amazon, finally got the 15600 mAh model, was tempted to get the 18000mAh, but it wasn't much more capacity for a lot more cash. It was $49.99 +$24.99 Shipping USD for a total of $74.99 USD.
There is nothing particularly special about this unit, other than good quality battery cells and what seems like good efficient charging/discharging. It has two 2.4A USB Charge ports and a small LED light, plus four stage led battery level indicator. Weighs about 360g.
I put it to good use almost straight away, with a five day self contained cycle trip (Alps 2 Ocean Trail) to keep my phone charged (HTC One). I drained my phone almost empty each day as I was Using a GPS cycle tracking program, and also used the phone for a couple hours each night while it was charging to use Amazon Kindle Cloud. Just the charging from each day's use alone should be about 12k mAh and it still had a bit over power left at the end of the trip so I'm pretty confident in it's mAh rating isn't exaggerated.
Due to the large rating it takes a fair bit of time to charge these up from empty, I only had a 1A charger available, but it can charge at up to 2A providing you have an adequate charger.
For anyone looking to keep phone/tablets/gopros charged on the go would recommend this product to anyone, particularly as there doesn't seem to be many good quality ones on the NZ market. They also have other sizes from 6000-24,000mAh. I am in no way affiliated with Limefuel.