It appears that part of the film will be shot at Harcourt Park in Upper Hutt, in the next couple of weeks. It appears to be a small battle scene, as they have bought in a number of trees which have been cut up to reflect lightning strikes. There are also a couple of large trees that have been mounted on pivorts, so they can fall after being hit by lightning.
I suspect it may be when the ringwraiths attacked Gandalf at Weathertop, thats the only occassion I can recall large pyrotechnics outdoors in the movie. If anyone has a digital camera you can pretty much just walk in and take some pictures. I was shooed off by a security man, but its a public park so there are no access issues at the moment.
He said it would all be gone in a couple of weeks, so I assume the filming will be taking place fairly soon.