"Enterococci and faecal coliform bacteria in the water are measured in cfu, which stands for 'colony forming units'.
"On occasion onshore winds will affect cfu levels close to the shore. Because it takes two weeks for these tests to be processed, by the time the council receives the results the incident has passed."
Fell said it was crucial to note that over the course of the year the median level was required to be below 35 cfu per 100ml and during the reporting period Whanganui District Council's recorded median was a very low 4 cfu per 100 ml.
During December, January and February, periods referred to in Radio New Zealand's story, the council took weekly recordings at six different locations, as required.
Horizons Regional Council regulatory manager, Greg Bevin, said Whanganui District Council's wastewater treatment plant resource consent with Horizons covered a multitude of parameters and consent conditions.
"With the exception of faecal coliform, by all other accounts Whanganui District Council has been compliant," Bevin said.
"We are currently in discussions with Whanganui District Council with regards to their faecal coliform readings, which recently show that compliance with this standard has been achieved 74 per cent of the time as opposed to the required 90 per cent."
Whanganui District Council's senior wastewater engineer Tony Hooper said it is common for stormwater infiltration to overwhelm the network during heavy rain.
"During these times we discharge partially treated wastewater, screened to remove solids," Hooper said.
"This is allowed by Horizons Regional Council, in line with our consent to divert screened wastewater directly to sea once flows to the plant reach 1120 litres per second. Dry weather flows vary from 200-500 litres per second; wet weather increases flows by four times."
Hooper said the occurrences are included in the council's annual consent report to Horizons.
"For context, when raw sewage arrives at the wastewater treatment plant, it has between 890,000 to 4,800,000 cfu per 100ml. The highest single test result this year was 830 cfu per 100ml at South Beach. This was not caused by network infiltration due to heavy rain – rather, it was caused by onshore winds blowing material back towards the beach causing a temporary accumulation."
In "normal times" wastewater is discharged at the pipe outlet 1.8km from the beach and it will disperse, Hooper said.
"But if there are strong enough winds blowing towards the beach from offshore it cannot go any further than the coast and will accumulate until weather conditions change."
