Rainbows End is the biggest theme park in New Zealand. But having said that, don't go there expecting huge sizes - Disneyland it isn't!! What you will find is a fair-sized, clean theme park with mostly average rides, and a couple of really good ones.
The rollercoaster is quite long, with a couple of good corkscrew turns in it, and the pirate ship is just as you would expect it. Other rides include mini golf, dodgems, go-karts, log flume, bumper boats, gold rush train, and the two big rides there - Fear Fall (you are sat on a bench, hoisted up 600 feet, and then dropped to the floor very quickly!), which is a really good ride, and not as scary as it sounds - also, when you're at the top, you have excellent views of Manukau for the couple of seconds before you fall!!
And also the Motion Master - basically a big simulator of a spaceship, which was really good fun and very well done. There are plenty of places to buy food, but if you want to take your own, leave it in the car and get your hand stamped so that you can go out to the car park and get back in again - much easier than carrying it with you all day.