One of Whanganui's landmark and highly visible tourist spots is looking decidedly off colour.
A bright green algal bloom has spread over most of Virginia Lake, but authorities say it poses no health hazard.
Barry Gilliland, Horizons Regional Council's water quality spokesman, said the algae had appeared because conditions were ideal for them to grow.
Mr Gilliland said Horizons had been looking at weekly samples from Whanganui District Council since November last year.
"Inevitably, the cause of a lake going green is some kind of algae in the water taking advantage of environmental conditions at the time."
He said algae were simple plants that used nutrients and sunlight to grow, and when these were plentiful a population explosion could turn a lake green.
"That's probably what has happened in Virginia Lake. Whatever the cause, I expect the algae to subside as they run out of nutrients in the water."
Mr Gilliland said the lake had experienced blue-green algal blooms in the past and these posed the biggest risk if the water and cells were swallowed.
He said if blue-green algae were present in any lake, it shouldn't be used for drinking or swimming, or, to a lesser extent, for boating.
"And depending on the [algal] species there may be a risk to dogs, but if the Virginia Lake reserve is dog-free then the risk for dogs should be very low indeed," he said.
There should be no need to stop people walking around the lake because there was a low risk of them coming into contact with the water.
Mr Gilliland said other district lakes were being monitored, including those the public could access, such as Wiritoa and Duddings Lake.
"Blue-green algae have been present at densities close to recreational guidelines from time to time, but health risk advisory signs were only required to be posted at Lake Wiritoa for two weeks prior to Christmas.
"Since then the density has been within guidelines and, based on previous experience, I fully expect they'll remain so for the rest of the season."