Hot temperatures have turned some of Napier's urban streams to bright shades of green, with an unpleasant smell to match.
Two of the most affected are the Plantation Waterway stream along Riverbend Rd and the Old Tutaekuri Riverbed stream.
St Patrick's School principal Jurek Wypych said the smell wafting over to the school from the Plantation Waterway stream was unpleasant.
"It does affect us, particularly when it is low and there is no flow of water. Summer is the worst season but there are issues throughout the year.
"There is often rubbish in the creek as well as a build-up of weed that is not regularly removed or cleaned.
"The reserve area is well maintained by the council but unfortunately the creek spoils that," Wypych said.
Hawke's Bay Regional Council and Napier City Council have responsibility for various parts of the city's streams.
The councils both told Hawke's Bay Today they were working together to enhance stream health in Napier.
HBRC said November's water temperatures were at their highest since testing sites were established in the mid-late 1990s.
"This would have exacerbated water quality problems in urban waterways."
A spokesperson said green growth in the Plantation Waterway stream was duckweed.
"The picture is typical of what a key problem is in low-land waterways," said Hawke's Bay Regional Council senior water quality and ecology scientist Sandy Haidekker.
A lack of vegetation shielding the stream from sunlight causes algae and aquatic plants to grow to a "nuisance level" in high water temperatures, Haidekker said.
Planting more trees is a potential solution but this would have to be done carefully to ensure vegetation would not cause flooding in heavy rain.
Haidekker did not have current data on whether the stream is healthy for humans but said she suspects there may be critical E. Coli concentrations in the water due to ducks and dogs in the area.
"The plants themselves are not unhealthy, the ducks love feeding and floating on the duckweed."
The smell could potentially be that the duckweed is blocking the flow of oxygen between the water and the air causing a sulphuric smell to be released, she said.
Residents have also raised concerns about the Old Tutaekuri Riverbed stream which one told Hawke's Bay Today "has been a disgrace for years".
"In warmer weather it is necessary to take a deep breath, then scurry across the bridge before breathing again."
Napier City Council's manager of environmental solutions, Cameron Burton, said the problem with the stream was likely nutrient growth that had been exacerbated by "consistently warm temperatures in low velocity waterways".
Contamination such as dog poo, car washing chemicals, sawdust, rubbish and dirt entering the stormwater system also contribute to the issue, Burton said.
"The odour is caused by the natural substance starting to break down naturally," he said.
The algae is cleaned from the stream and taken to landfills every year in the first week of December and will be occurring later this week, he said.
Burton recommends humans and dogs do not come in contact with the stream and said there may be other toxins underneath the algae which they are unaware of.