The alpkit Gourdon 20 might be the ideal day pack. Its light, durable and cheap. But the main thing is that it is waterproof. Alpkit have taken a 20 litre dry bag and added a decent suspension system to it – padde shoulder straps along with a chest and waist strap. The bag also features 2 mesh pockets which will accomodate water bottles. a bungy system for securing extra clothing and a removable back pad / hydration pack area.
If you are unfamiliar with dry bags, you simply roll the top down 2-3 turns and clip the straps together. This style of bag is popular amongst divers and kayakers – they can usually stand total immersion and therefore should cope with New Zealand’s West Coast 
.
For £17.50 I feel this represents a bargain and totally beats the usual daysacks you see (like the integrated one on the ever popular karrimor rucksaks). Its around 600gm so could easily pack away inside your main pack as well.
Alpkit also do 25 and 30l versions, but these as yet do not have the mesh pockets and Bungee on them.
Its great to be able to walk around in the rain without worrying about cameras, phones etc etc.
Grab one here – Alpkit Gourdon 20