Here is an excellent article on the state of play of the Didymo problem in the South Island of New Zealand.
The sad part is that the situation in New Zealand is a graphic illustration of the ‘portability’ of all sorts of pests because of the comparative ease of travel. It is believed the introduction of Didymo in New Zealand was by way of an American angler, or returning Kiwi angler – the Didymo strain in New Zealand is the same as that found found in the USA.
Is there a hope of eradicating the pest – not in the foreseeable future, however there is growing evidence that Didymo does fluctuate with water levels and increased flows.
The good news is that the impact of Didymo on river life, especially the insects trout feed on, are not as badly impacted as first thought, but are actually thriving.