Shitty slimey rock snot

When the news first broke about Didymosphenia geminata aka Didymo aka Rock Snot (a fresh water diatom) having been found in the Mararoa River and Lower Waiau rivers in 2004 there was a fair bit of hand wringing and teeth gnashing. Within months, outbreaks had been noted in numerous waterways in the lower South Island and MAF released its' Clean Check Dry campaign. But still to this day its all a bit toothless and ineffective. Check this out:



And now for the map of how it has spread:



That it has not been found/spread in the North Island is just a matter of random luck, it has to be. Or another possibility is that the South Island is paradise for Didymo, and paradise must be blighted.