Water is also monitored for blue-green algae/cyanobacteria. It should be avoided if there are more than 1.8 cubic millimetres of toxic algae per litre, or more than 10 cubic millimetres of non-toxic algae per litre.
Council staff warn people and animals to steer clear of bright green blooms and scums on lakes, and of black, slimy, musty smelling mat-like growths on river stones.
The two websites are only updated weekly, and heavy rain can change swimmability within hours. Generally the swimming is good if water looks clear and clean and there has been no rain for two or three days.
People swimming should also watch out for other types of hazard, such as submerged logs in rivers and rips in the sea.
Swimming water quality in the Horizons region is below the national average, chairman Bruce Gordon says, but is improving faster than the national average. On December 18 three Whanganui coastal streams and two lakes had health warning signs posted, and there was also a no-go signal for swimming in the Mangawhero River at Raukawa Falls.
Work the council plans could lift the swimmable length of region rivers from 43 to 60 per cent by 2030.
++ This summer people can go in the draw to win a $1500 adventure package from Horizons by taking a photo of their favourite swimspot, adding the hashtag #swiminourregion and uploading it to Facebook, Instagram or Twitter.
