Permaculture in New Zealand

I have a grass paddock that I want to ultimately turn into a food forest. I want to plant a green manure crop along with various nitrogen fixing plants asap. How do I go about converting the grass to green crop? Do I need to eliminate all of the grass first? lots of mulch? is there a green crop I can use that will grow within and take over the grass? Any thoughts on the best and cheapest way to get started would be appreciated.

Thanks

Rob