On the Trail Kit

You need to carry enough tools and bits so you can hobble home on your wounded bike but not be laden down like a US Marine.

The Essentials

• Helmet - Make sure it fits properly, ie. fits on your head rather than sitting on top of it and has minimal movement from side to side. Check that it won't get pushed back to expose your forehead or fall forward on bumpy descents leaving you temporarily blinded. The straps must be adjusted for a firm but not suffocating fit. Visors look cool, keep the sun off your hooter and help deflect unwelcome bits of undergrowth. It's vital to replace your helmet after any major impact or when it is visibly deteriorating. Most reputable brands offer a crash replacement deal (remember to keep your receipt). 
• Gloves - When you involuntarily dismount your reflex reaction is to protect yourself with outstretched arms - your palms take the brunt of the fall so you need a quality leather or synthetic leather glove to prevent severe gravel rash. 
• Cycle Shorts - You just gotta have some padding between your bum and the seat. Your choices are a conventional 4-way stretch lycra short or a baggy cycling sort with integrated padding.

The Jolly Important Things

• Mountain biking often takes you out for long days away from home. It's important to be prepared for the variable conditions NZ's climate can toss at you. 
• For winter - a thermal riding top made from micro fleece or polypropylene. They don't absorb water so you avoid chilling after sweating on hard hill climbs or if you get caught out in the rain. 
• Cycling tights or long johns - toasty warm and help prevent injury to dodgy knees. 
• Wind/rain shell - keeping the rain at bay is obviously important. Cold winds are a cyclist's major enemy - you need to block these out to avoid the chills on exposed tops. Many cycle specific jackets turn into themselves to form bum-bags when they're not being worn. Consider hi-visibility colours if you're planning to ride on the road. 
• Thermal hat and gloves (micro fleece, polypropylene or similar) - most helmets and gloves are designed to keep you cool in hot weather, not to protect your extremities from plummeting temperatures and icy winds. 
• Eye wear - handy for deflecting wind and bugs on those rapid descents, blocking out those nasty UV's, and for avoiding mud-in-ya-eye syndrome. Plastic is lighter and safer than glass, although more prone to scratching. 
• Lightweight lights - that clip onto your bike or clothing are worthy insurance against unplanned time on the road. It's not uncommon to be caught out later than you intended.