Surfers are concerned they will get sick when largely untreated water is pumped into a popular New Plymouth surf break.
The New Plymouth District Council is upgrading its wastewater treatment plant and is taking an aeration basin out of use from Monday for the next four months.
The change means only half of the city's wastewater will be properly treated.
The rest will be screened to remove large material and disinfected before going out via the Waiwhakaiho outfall.
Water and wastes manager Mark Hall says there is a slightly higher risk of those who eat shellfish from the area getting sick and of skin irritation for water users.
The outfall is at the well-used Waiwhakaiho surf break and near the popular Fitzroy and Bell Block surfing and swimming beaches.
New Plymouth Surf Riders Club secretary Allen Pidwell says surfers have become ill while using the break before.
"The outfall is right next to the Waiwhakaiho break, which is one of the ones protected under the New Zealand Coastal Protection Statement, and it's used throughout the year by a number of surfers. The concern will be that there will be an increase in illness."
Mr Pidwell says the matter will be discussed at surf club meetings this week.
Fitzroy Surf Lifesaving Club president Todd Cations-Velvin says the timing of the upgrade is bad going into the summer months.
He is worried about increased health risks for ocean users and says the council could have done a better job of informing them.
Mr Cations-Velvin says he heard about the plan only on Monday.
He says the only saving grace may be the predominant westerly flow, which could take the discharge away from local beaches.
The council says the upgrade needs to be done during summer when waterflows are low.
Mr Hall says in winter more water goes through the plant.