The foul-smelling black sludge clogging up the Waipu Cove rivermouth is not human or animal effluent, but it's still not a good idea to swim in the area.
The Whangerei District and Northland Regional councils want to quash rumours that the effluent-smelling stench and black sludge is sewage.
The councils are working together to tackle the problem, which has been the subject of many residents' calls to both councils over recent weeks.
NRC environmental officer Jean-Charles Perquin said quite a few people had wondered what it was but WDC testing had confirmed it was not sewage, but seaweed and algae which, in the stagnant stream, had started to rot.
While this natural sediment was common in this stream, "it's been a lot worse this year", Mr Perquin said.
To rule out sewage, WDC staff have extensively checked the sewage system and have taken weekly samples of the water to test bacteria levels.
"Rotting will have bacteria as it is part of the decomposition process - but it will not hit the 3000 to 4000 e-coli bacteria per hundred million that sewage would show," he said.
Regardless of the cause, it is considered unsafe to swim.
"But Waipu Cove - the beach itself - is clean, and has been clean for the past five weeks [that it's been tested]," said Mr Perquin.
And digging away the banks of the stream to allow the sediment to drain into the ocean could make it unsafe to swim at the cove. "It's better to keep it contained - it's going to go away anyway."
WDC waste and drainage manager Andrew Carvell said the storm before Christmas could be one of the culprits.
"Algae and seaweed got washed in there as well as sand which created areas where water can't escape and it's been decaying - hence the odour," he said.
"There are a lot of people who have complained. Some people who live out here say it's the worst it's ever been. If it was sewage, the counts would be a lot higher and it would be a lot more localised. This is virtually right up the river nearly to the bridge."
Waipu Cove surf lifesaving club president Rick Stolwerk said that while the smell had been really bad at the beach the intensity seemed to have subsided.