Kiwi fishermen stumped by 'taniwha egg' that washed up on Mangawhai beach

Kiwi fishermen were left stumped by a pair of sickly yellow objects which washed up on the beach near Mangawhai.

A mysterious egg-shaped object which washed up on a beach near Mangawhai/Langs beach. Source: Darius Lindsay

Posting earlier this week in a New Zealand fishing group on Facebook, Darius Lindsay asked his fellow anglers, "can some please tell me what the **** this thing is?"

Possible answers from the punters included a "dragon's egg", a "coronafish", "5G" and a "taniwha egg".

One person suggested that it could be some type of sea sponge, with another not too far off the mark in suggested it could be a sea tulip.

NIWA marine biologist Dr Michelle Kelly took one look at it and identified it as either polymastia crocea or polymastia aurantia - most likely the former.

It's described as a "cushion or loaf-shaped sponge, typically 10-20cm in diameter and 10 cm thick".

A graphic of polymastia crocea from NIWA's Splendid Sponges guidebook. Source: Supplied

"Texture soft, compressible, elastic, velvety to the touch," NIWA's Splendid Sponges guidebook says.

"Colour in life bright lemon yellow or turmeric yellow, internal colour creamy yellow."

Dr Kelly said the sponges are sometimes dislodged by heavy wave action and are rolled around on the sea bed before being cast up onto beaches.

They're found mostly in along Bay of Plenty, Taranaki, Northland and Hauraki Gulf beaches, but a small number have been seen around Wellington's south coast.