Ranskye, I'm in favor of pretty strict environmental protection. Unfortunately, when technocrats get involved in making regulations to cover a variety of situations, things get muddled and you inevitably end up with some provisions which are ludicrous or counter-productive. In the US, there are federal provisions for aquaculture but they do not kick in until the production level is pretty high. Most regulation is done at the state level. For KFG, Georgia has an aquaculture license program although there are very few provisions.
http://georgiawildlife.dnr.state.ga....=330&txtPage=1
Hawaii has much stricter provisions, but none of the US states are as strict as Australia.
KFG, net pens are... well, pens made out of netting. Turning fish loose in those large ponds would become problematic when you want to retrieve them. I would be afraid to rely on your friend's technique of throwing a cast net on them at feeding time. However, tosai and older could be housed in net enclosures.
The advantage of the net pen is that you would utilize the large water volume, but still be able to retrieve the fish when needed. The pen would have four sides and a bottom of netting. Poles stick in the pond bottom, floats or a permanent structure would keep the top of the pen a foot or two above water. They would also need a bird net cover if you have large herons. To remove the fish, the netting is pulled up and the fish are scooped up with dip nets. About 20 ft x 20 ft is as large as one or two people would want to handle.
Ideally, a paddle wheel would be set in the main body of the pond with the water flow pointed at the net pen, or a row of net pens, to insure there is circulation through the pen. A second-best option would be to put one or several air stones inside each pen. If the koi are being fed, I would be reluctant to operate without some sort of supplemental aeration.
The net pen can be a deep as you want it to be, up to the depth of the pond. You want koi to have access to the bottom of the pond where they get valuable nutrition from poking around in the mud. A net pen which sits on the bottom is almost as good as a bare-bottom mud pond. Use a net coating at least on the bottom to make the part which is in contact with the pond mud last longer.
Nylon netting is used because it lasts the longest but is still flexible. A stiff plastic material would be too hard to cinch-up when removing fish. You can buy the material and sew the net pen yourself, or have them made by a company like Memphis Net and Twine or Nylon Net Company.
-steve hopkins