Rowan has it right in regards to NZ hills and mountains, most people use the term backpack as a generic term for a bag which goes on your back. New Zealand mountains and back country kill with ease, so if you plan on walking them be properly prepared.
Apart from that the basic rules are - pack each day separately. Double socks and underpants per day (nothing like a sweatrash to make things memorable). Waterproof, waterproof, waterproof. Pack light, pack prepared. Keep a small container with wax dipped matches a windproof lights, mirror, tin foil, ref whistle and a small plastic bag. Seal this in wax and keep on you at all times. Pack rollon insect repellent and rash cream.

My kit is a little unusual mainly because of the type of conditions we go into. Tampax for puncture wounds, suture kit, cow mastitis ointment and applicator, 5 days worth of antibiotics suitable for chest infections etc etc.