Regional ratepayers will also shoulder Horizons' cost for bringing the prosecution.
In January 2017, human wastewater from a district council pump station entered the Mowhanau Stream after a Powerco pole fuse failed, prompting the prosecution.
Yesterday, Fell accused the regional council of grasping at straws, saying the legal action had achieved nothing.
He said a low volume of treated septic tank wastewater had overflowed from a pump station, and the district council had responded by upgrading the pump station to include a continuous power supply and the ability to connect a generator in the event of an external power failure.
These works were completed in March 2017, but council received a notice of prosecution from Horizons four months later.
"The result is that Whanganui District Council has been found guilty of one offence of discharging a contaminant, which we accept occurred," said Fell.
"There are no requirements from the court to do further work or to change any processes and procedures.
"I believe they're scrambling to demonstrate some kind of effectiveness at the moment and are using our case to shore up some credibility.
"Their action against Whanganui District Council has achieved absolutely nothing."
However, Horizons' regulation manager Nic Peet said the regional council weighed carefully whether or not to prosecute.
"Horizons is conscious that the prosecution process comes at a cost to both Whanganui and regional ratepayers and we've done our best to keep these costs down.
"However, the issues were significant enough that they needed to be brought in front of the court."
Peet said the stream is a popular swimming place for young children.
Fell said residents could be assured the Whanganui council took its responsibilities seriously with respect to the natural environment.
"We responded quickly to the pump station issues and there has been no repeat of the incident."
In two other court actions brought by Horizons, Whanganui's Land Meat has been prosecuted for discharging wastewater into the Whanganui River in March this year.
Land Meat, owned by Affco New Zealand, processes cattle and pigs at a meatworks near Whanganui's river port. It will be sentenced on August 21.
Horizons also prosecuted Le Poulet Fabuleux, a chicken rearing operation near Himatangi.
It had been discharging effluent and washwater into a farm drain that ends at Lake Koputara.
The poultry business was fined $57,000 in Palmerston North District Court.
There are two other prosecutions still in the pipeline.
