Red seaweed washing up on beaches on the Coromandel Peninsula is causing a stink, but councils say there's no risk to the public.
But beachgoers are being warned the stench is likely to get worse before it gets any better.
Testing by NIWA today confirmed the seaweed was just a harmless native red algae known as Spyridia filamentosa, or hairy basket weed.
"While this is a completely natural event, masses of this non-toxic seaweed being washed up can cause an unpleasant odour so we're keeping a close eye on what's happening," the council's Coromandel zone manager Tonia Clarkson said.
She said the smell would likely get worse as the seaweed started to break down.
"We're hoping nature will deal with it but, because of the odour risks, it is important to monitor it and we're keen to hear from people spotting large accumulations of it on their local beach."
Significant amounts of the seaweed have also been washed up on beaches in Northland.
"Over summer it's actually common for this type of seaweed to accumulate in large clumps.
"Those clumps are normally found offshore and further north of the Waikato region, but the recent strong winds we're experiencing courtesy of El Nino have brought them our way," Ms Clarkson said.
Council staff are out checking beaches and people wanting to report sightings of the red seaweed can call the council on 0800 800 401.