Krill washed up in huge numbers at Pukerua Beach on the Kāpiti Coast.

A "rank" and "stinky" fishy smell is coming from swarms of krill that washed up on Kāpiti Coast beaches several days ago.

Krill was thick enough to change the appearance of the sand, and spread as far as 10 kilometres down the coast.

Residents first spotted it on Thursday, with one comparing the smell to shrimp paste or "like the sea, intensified".

Now the krill has sat out in the elements, the descriptors aren't so gentle.

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Social media commenters have described the stench as "rank" and "stinky", and say they smelled it from main roads on their way home.

It's very fishy and unpleasant, said Andy McKay, the convenor for community conservation project Ngā Uruora - Kāpiti project.

"The birds are pretty happy at the moment," he said.

He'd been in Waikanae with his kids on Saturday and seen gulls feeding off the krill, which was bleached white after a few days in the sun.

The smell was awful, he said, and he could still smell it on his kids that evening.

He'd also seen comments from people who caught a whiff while stuck in traffic on State Highway 1 between Pukerua Bay and Paekakariki.He expected the krill to clear up in a few days' time and said the wash-up was good news in a way - it shows there are plenty out at sea too.

JENNY CLARK After days in the sun, the krill is starting to stink.

The Kāpiti area is in line for the D'Urville current, which can sweep things onto its shores, he said, such as the blue jellyfish-like velella from 2017.

Kapiti Coast deputy mayor Janet Holborow said the krill had "a very distinctive odour, that's for sure".

On the plus side, they're food for local marine birds, fish, and kororā (little penguins).

Heavy on-shore winds or a strong current would be the most likely reasons for the krill washing ashore, a statement from the Department of Conservation said.

"There look to be several species involved, the most common one is probably Nyctiphanes australis," technical advisor for marine ecosystems Emma Hill said.

The volume of krill - a zooplankton - is probably because of a phyto-plankton bloom in the area, which meant the krill had plenty of food, she said.

And while big washups aren't common, they aren't unusual either.

For example, "lobster krill" washed up on Nelson's Tahunanui Beach in November 2018.

That kind of thing is more common along the west coast because of the prevailing westerly winds.