Welcome to bikes.SE ..... (You may guess form the links I am also a NZer)
I am not a fan of upgrading parts on bikes. Normally a bike is made up of matched parts in terms of price and quality. Upgrading your Hardrock with say SID forks would be a complete waste of money - the bike will still be a Hardrock... New forks will cost $500 or more, plus installation, which is more than your bike is worth. Ask yourself, who is likely to know best, you or Specialized?
The Hardrock is a good entry level bike. It is designed for off road use - it will not break or get damaged any more than another bike. Better parts will not significantly improve the durability of the Hardrock. What happens with better parts is smoother operation, lighter weight and better handling -- overall a more enjoyable riding experience, especially if you are a skilled rider. With better shocks you get more options for fine tuning, better handling as they will hold the wheel on the ground for longer. (I am assuming XC, not national down-hill use).
I suggest you don't worry about parts. Find a good local bike shop (Checkout vorb.org.nz forums as it's a commonly asked question) and get them to look over the bike and maintain it if you don't know how. A good bike shop will even give you advice on what you can do yourself.
As far as the ride goes, my experience is you will see everything from 10-15 year old bikes, a few low-end and plenty of mid-end, and a few fancy $8k carbon bikes at the start. Don't be intimidated. After marshaling at the finish of an 80km event I can tell you that looking at the value and quality of the bike is a poor way to predict the order in which they cross the finish line.
