Doesn`t sound right!
New Zealand have a very high standard, well Im not about to dispute that, however the standard required for a test is printed in black and white. You dont have to be above the standard, just meet it. I beleive that everywhere around the world the standard would be about the same. Minima`s are minima`s no matter what altitude they are at, same with DME distances.
As for 20ft, well you are in fact allowed to go below the minima as long as you take immediate steps to correct it. Immediate. A small thermal would cause you to loss 20ft quicker than you can react to do anything about it. If you then fly to the MAP 20ft low then, back to school. I descended below the minima because the engine was shut down and the aircraft couldn`t maintain altitude, the engine had been feathered 3 to 5 to the live and all that crap. I couldn`t be failed as I was following every procedure taught to me. Once the ATO said go around he quickly gave me the engine back as there was nothing else to test me on except how I would crash the thing!