Algae believed to be Spyridia filamentosa are responsible for a thick red layer of goop that has formed on some of New Zealand's most popular summer beaches.
A number of beaches in Northland have been affected, including Waipu Cove, Langs Beach, Oakura and Te Haumi.
Langs Beach resident Colin Harvey said the algae were so thick at one end of the beach, at least two tractors had gotten stuck in the red sludge.
"They try and manoeuvre around it and it just sucks them in. It's incredible, it just turns the sand to glug."
Mr Harvey said that at Langs Beach, the algal sludge had been accumulating on the shore for about a week.