It's just been one problem after another for New Zealand's Lake Omapere, according to this article from The New Zealand Herald. Back in 2000, populations of oxygen weed (an American invasive, either Egeria densa or Elodea canadensis) were so large that they were smothering anything trying to grow at the bottom of the lake. To combat the weed's spread, officials released over 40,000 grass carp (Ctenopharyngodon idella), known to many as an invasive fish species. Oxygen weed levels have dropped, but the decline appears to be the result of a huge algal bloom across the lake surface. Now the algae-rich lake water is contaminating local rivers, and officials are warning people to avoid even coming into contact with it. The government has dedicated more funding towards studying the problem, hopefully from a nutrient-level standpoint.
Thanks to S. Pagad for sending in the link to this story.