The algal bloom at Virginia Lake is not toxic but the Wanganui District Council is still warning visitors to keep clear of the water there.
Signs went up around Virginia Lake just before Christmas, asking people to wash their hands if they came into contact with the water.
Council infrastructuremanager Mark Hughes said testing had shown that the blue-green bloom was not toxic. Further testing will be done later this week, he said.
Mr Hughes said this year algal blooms have been much worse and widespread, with the whole country affected and several waterways closed to the public.
"However, Virginia Lake is not used for recreational swimming, boating, fishing or dog walking, so there is not a public health issue.
Horizons water quality expert Barry Gilliland said algae counts had fallen at Dudding Lake and were within acceptable guidelines.
Algae counts were still high at Lake Wiritoa but this algae was not toxic, he said.
"The usual advice [is] to mix the surface water up by doing a few circuits of the lake in a power boat before undertaking activities like water skiing. Although no surface scums were observed when sampling, this is simply a precaution to mix any algae into more of the lake water if it is concentrated near the surface, especially first thing in the morning," Mr Gilliland said.
He said blue-green algae thrived in warm temperatures and stable water conditions.
Four of the six swimming spots Horizons tested for water safety had come back clear in the latest round of testing. They are Kai Iwi and Castlecliff beaches, Mowhanau Stream and Whanganui River at the city bridge. The water at Lake Wiritoa and Dudding Lake was acceptable.
