For two or three day mountain bike trips the trick is to take the least amount of stuff that you can safely get away with. This invariably means omitting a few comforts but is a worthy sacrifice... tackling wonderful singletrack while being loaded like a packhorse is not a good time.
If you land in the cactus while in the back of beyond you've gotta yell pretty loud to get help. Travelling light doesn't mean foregoing emergency stuff and you do need to make conservative decisions that reflect your ability to deal with the unexpected. Typically this involves researching the area you're heading into, timing the trip with a decent weather forecast, making sure your bike is fit and healthy, riding a little less on-the-edge than usual, and having the 'huevos' to turn back if things aren't going to plan. Mobile phones have limited coverage in the NZ backcountry - but are occasionally useful to holler for help, text your mates or call for pizzas.
Cart your stuff in a lightweight pack - about 25-30 litres capacity. The Osprey Escapist is our pick. Panniers are great for off-road touring, but don't cut the mustard on technical track, grovelling through undergrowth or when carrying your bike. That said, a Freeload Rack with a dry bag strapped to it can be an excellent option in open country. Tents and sleeping mats are millstones best avoided. Plan your trip around huts and survive the night with a compact 150-300g down sleeping bag (300-500g in winter) coupled with a lightweight down jacket for stooging around the hut. Not always a cozy pit, but good enough after a hard day in the saddle.
Even in summer it can get cold and wet. Pack a pair of Daddy Long Legs tights, a Baked Alaska or Pushover thermal top, perhaps a Submerino thermal base-layer, an anti-Cyclone or Antidote rain jacket and Helter Skelters over-trou'. Plus a Baked Beanie for ya noggin' and some lightweight polypro gloves to wear under your cycling gloves. I survive on a single riding top and pair of shorts. Washing your nether regions in the evening helps avoid saddle sores.
You'll need a lot of space for food. Choose wisely - check out the Art of Food and Bikes. Pre-cooked avoids the need for a billy and stove but is a bit austere. If you're in some dodgy place overseas, then a water filter is good insurance. It's worth paying extra for a high capacity model.
Apart from the standard trail tools, it pays to pack some spare parts. Depending on the terrain carry a few spokes, a folding tyre, rear hanger and/or derailleur. A headlight and flashing tail light are handy if be-nighted or suffer an after-dark road-bash back to the car.
Surprisingly, your body is even more precious than your bike. Pack some heavy duty painkillers, arnica, anti-inflammatory and antihistamine tablets, plaster and dressings for nasty falls, a crepe bandage and survival blanket. You can score a ready-made kit from St Johns Ambulance or Aide. If you haven't done so already, shuffle along to a basic first aid course.
At the risk of stating the obvious carry a map, compass and the skills to use them. If your route crosses private land then ask the land owner's permission first... and try to glean local knowledge from them. A GPS is a useful toy to know exactly where you are, how far you climbed, how you got there and all sorts of other train spotting data. For the paranoid or on truly death defying trips take an PLB or Spot Tracker like they use in multi-day adventure races. Setting one off is like blowing a whistle that the whole of the Southern Hemisphere can hear. The New Zealand Mountain Safety Council regularly hold Outdoor Safety Courses in First Aid, Navigation and even Risk Management. All handy tools for your resumé.
Have fun and be careful out there.