farm work in NZ

Advice please.

I am coming over to NZ in Jan on a WHV and as a London city worker I am keen in try something different on my travels.
I like the idea of working on a farm and have seen the websites Frenz and WWOOF...
A few questions:

Should i wait untill I arrive to book
What areas, regions would any one suggest...favourite spots to do this sort of thing?
websites to check out

As a lone female traveller I'm overly causious.
Apologies for my ignorance in advance,

Thanks

Did WWOOF awhile ago. Found it best to book a few days in advance.
In terms of regions to go??? Our best experiences revolved around the families we stayed with rather than a particular farm.
Here were our favorite 2:
Wilderland located in Corramandel- best "farm" experience that we had by far and it was somewhat shared by a few other travellers. you can actually websearch Wilderland or wilderland NZ and find some articles.
Our other favorite was at the TreeCrop farm in Akarora- believe on the web also.
The sleeping accomadations can be a little rough at times. We had one in a really junky campervan that probably hadn't been cleaned in years. Was a little gross. Otherwise, WWOOF was a great experience in terms of meeting people, staying with families...
Enjoy

Ive never done WWOOFing but before i even read keewekid's post i was going to recommend a place in the Coromandel called Thames and anywhere in Akaroa, below Christchurch (utterly beauitiful). I stayed in a Thames (well up in the green hils really as it was just by a load of walking paths- not down in the town) back in October/November 2007 and they had a couple of WWOOFers who semed to be having a wonderful time. They were a young couple from France who were covering costs of thier travels around NZ by doing the farm work (they also helped out in the kitchen). They ate dinner with us and we all sat round eating, drinking and chatting. I could really go on about how beauitiful the farm there was etc but Im worried the couple will suddenly be overwhelmed with requests to work there. If your particularly keen to know the exact place feel free to message me. NZ is just full of lush green hills and sheep!

I WWOOFed in Queen Charlotte Sound on the top of the south island and I can highly recommend it. The site Backpacker Board is a good place to look for short term WWOOF and farm jobs. I personally worked at Lochmara Lodge and it was awesome. In fact I've not been back to London now in 2 1/2 years!! (I now live and work in NZ).