The beauty of the supply management system is its stability. When production, imports and prices are not regulated, we end up with prices that fluctuate a lot. When they are low, small producers die, if I can put it that way. Market fluctuations invariably lead, as you mentioned so well, to a concentration towards the biggest producers.
That said, regarding New Zealand, to get through periods of low prices, it offers massive subsidies to producers, which we don't need to do here. This costs their taxpayers money and allows their producers to sell at prices below the cost of production, as the difference is made up by subsidies. This makes it difficult for our producers to compete if they are not subsidized to the same level, hence the importance of protecting them.
Every time supply management has been breached to open up 1%, 2%, or 3% of the market, all of that market share has been filled, because a subsidized producer is always going to undercut them, inevitably. That's exactly why we need to maintain the system, including the border barriers, because our producers are not supported by the government, unlike their competitors.