Why red seaweed has blanketed Auckland beaches

Thick red seaweed that has covered beaches on Auckland's Hibiscus Coast is a natural occurrence, experts say, but locals remain convinced they haven't seen it before.

Both Ōrewa and Red Beach have been covered in the hair-like seaweed since the beginning of the week.

The thick layering of seaweed was "beginning to smell" at Ōrewa Beach, Hibiscus Coast resident Scott Douglas said.

"It's been accumulating all week - it was worse when it first appeared in the water because it would stick to your body while swimming.

"I have no idea what sort of seaweed it could be. I've lived here for a year and never seen it before."

Members of the public contacted Biosecurity New Zealand about the seaweed, team leader of aquatic health Rissa Williams confirmed.

"This is a type of polysiphonia, a common genus of seaweed in New Zealand," Williams said. "There's been similar reports at beaches in previous summers".

But Ōrewa resident Houshang Radkhou - who has lived in the area for a decade - said it was the first time he had ever seen seaweed like it.

"It's beautiful - I've never seen it like this before, normally any seaweed that has washed up is green. I did have a close look at it and checked inside and it looked green in the middle but red outside."

Fellow Ōrewa local Alan Flint agreed with Radkhou that he had not seen seaweed like it at the beach before but he said it was "a bit of an eyesore".

"I'll be glad when it finally gets washed away. It's looking better than it did a couple of days ago though, the whole beach was covered."

The tide had pushed the seaweed further up the bank, Flint said, and it was now piled up by the steps which lead on to the beach.

Algae goes through blooms, lead environmental scientist at Auckland Council, Dr Megan Carbines said, and warmer sea temperatures over the past two years had increased growth.

The accumulation of the red seaweed was likely due to that along with some onshore winds pushing it up, she said.

"The seaweed will disperse as the temperatures continue to cool and wind conditions change."

As marine heat waves and increasingly warm temperatures continued, Carbines said it was likely Aotearoa would see more of these events in the future.

University of Auckland associate professor of marine science Dr Nick Shears backed this up and said calm conditions and warm weather were perfect conditions for red algae to grow.

"We're not 100 percent sure why these events are becoming more common, but obviously we have had fairly rapid warming conditions over the last decade."