A nm is 1/1,000,000,000m, so 50nm is 50/1,000,000,000m, or 1/20,000,000m. Multiply by 500,000 gives 1/40m or 25mm. So, aye, if the 50nm information is correct, the holes would need to be 2.5cm. Larger holes than (intact) fishnet stockings, in other words. Now, how big is New Zealand and how big is an average fish?