Cases of norovirus were caused by swimming in Cardiff Bay, the city council has confirmed.
It follows concerns raised last week about the poor water quality in the area, which led to the controversial move of a water park attraction to Comeston Lakes.
One councillor, speaking at the council's environmental scrutiny committee last Thursday, said it was harming the ability for the Bay to be used by members of the public and businesses.
The fresh water lake, impounded by the Cardiff Bay Barrage in 2000, is fed by the River Ely and River Taff which have both suffered from sewage discharges in recent years.
Conservative group leader Cllr John Lancaster said: "Clearly we have a responsibility to the people who live around the Bay, the people who use it, businesses that want to be there, so surely there has to be some sort of pressure from yourselves [council officers] to work with the likes of NRW [Natural Resources Wales] and Welsh Water to address it."
The Cardiff Harbour Authority (CHA) publishes a daily water quality monitoring report on its website.
On the day of the meeting, Cllr Lancaster said he noticed four out of the six stations where monitoring takes place recorded a ‘fail’ for water quality.
“It’s not really good enough is it?” said Cllr Lancaster. “There’s a real danger that poor quality is harming the ability of the Bay to be used by the citizens of Cardiff and businesses."
“Surely there should be some sort of attempt to address that and not just say, ‘well it’s coming from up stream…it’s out of our hands’.”
Data published on the CHA’s website on Monday showed all six monitoring stations recorded a ‘fail’ for water quality.
Monitoring at Cardiff Bay has been taking place since it was impounded by the creation of the barrage in 2000.
A council report states the CHA sampled for viruses for many years, but stopped in agreement with Public Health Wales because none were recorded.
Cllr Jennifer Burke, cabinet member for sport, culture and parks, told the meeting that an incident management team chaired by Public Health Wales identified norovirus as the cause of a series of illnesses in the summer of 2024.
The report added: "The current monitoring programme looks for indicators of sewage contamination, therefore it is presumed that the source of Norovirus in the environment would be sewage related."
"It would be expected that there is some protection afforded by the current monitoring regime in respect to viruses."
Earlier this year, a research programme was initiated with Bangor University to look at testing for norovirus in Cardiff Bay.
In response to Cllr Lancaster’s comments, Cllr Burke said: "I don’t think it’s fair to say that we’re just throwing our hands up and saying ‘we’re not doing anything’."
"Cardiff Council are working in partnership and pushing back on Welsh Water and Natural Resources Wales."
"We’re doing what we can with what legislation exists and obviously there’s part of this that sits with Welsh Government and the leaders that they have available."
"There’s only so much we can do and we are at the end of the waterway so whatever we’re doing locally is making an impact, but we can’t combat what’s happening further up the watercourse."
In March, Vale of Glamorgan Council announced Aqua Park's move to Cosmeston Lakes as part of a summer trial, which ended on Sunday.
The water-based inflatable assault course first opened on Cardiff Bay three years ago and welcomed hundreds of visitors each summer.
In March, a CHA spokesperson said: "Recent summers have been very wet and this increased rainfall, and the higher river flows that results in, have increased the number of failed tests."
"This in turn has impacted on the very limited number of activities allowed in the Bay which involve full immersion in the water."
"Cardiff Harbour Authority has no control over the quality of the water that flows into the Bay and as has been well-publicised recently, water quality is an issue in rivers, lakes and other fresh water bodies across the UK."
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