Waikato District Health Board has issued a warning for Lake Ngaroto - near Te Awamutu - after tests found a dangerous toxin, cyanobacterium in the water.
And that presents a challenge for dog owners.
Andrea Pieper says she and her husband regularly take their dog Tika for walks around the lake now they will have to be mindful to keep their labrador on the lead and away from water.
"She loves the water so we won't let her go for a swim in the water if there is any sign of the toxic algae in it, because it's deadly for dogs, so yeah it's a pretty big concern for us."
Health Experts say the concentration of cyanobacteria is nearly double the recommended safe level for recreational use.
Cyanobacteria biovolume has risen to 3.2mm3/L at Lake Ngaroto. Biovolume is the measure used to decide when a health warning should be issued, with the cut-off value for a health warning set at 1.8mm3/L.
Medical Officer of Health Richard Wall recommends that lakes where the warning is in place should not be used for any activity that involves skin contact with the affected water
"The message is that there is a potential health risk there, people could come to harm and need be aware of that and stay away from the lake. If you are going to use it then be careful and to keep your kids away from the area and not to play on the edge of the lake in scums and things like that."
It can cause allergic reactions in humans, rashes, severe vomiting and diarrhoea. Long term contact can result in issues with the liver or central nervous system.
"It can be particularly more severe for young children perhaps or for people with medical conditions already who might be more likely to have problems from it, and the concentration of the toxins released from the cyanobacteria might vary around the lake," Mr Wall says.
The DHB says it is not an outright ban and it can't stop people from using the lake if they choose to - sailors and rowers say they'll continue using the lake water but shower afterwards.
The toxin could be present in the food chain
Scientists believe run-off containing fertiliser is one of the main reasons the algae is growing - because too many nutrients in the systems cause cyanobacterial blooms..
The Waikato Regional Council's Lake Scientist, Dr Deniz Ozkundakci, says they collect monthly samples from the edge of Lake Ngaroto.
"So the monitoring for cyanobacteria is related to recreational use of water so we're trying to sample the water where people actually get into the water.
But it is hard to get a full representation of cyanobacteria across the entire lake because we can't necessarily cover the whole lake surface all the time, we do have to assume that the sample that we do take is reasonably representative of the entire lake," Dr Ozkundakci says.
The Regional Council is developing new technology to detect algal blooms in real time. In two years they hope data from buoys in the lake will be able to quickly inform the Waikato District Health Board of potential health risks associated with algae.
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