"The birds had picked the eyes out but the flesh had not been touched," Wendy Morrell said.
There's something fishy going on in the Hauraki Gulf - thousands of dead mackerel were found floating in the sea near an island beach.

Wendy Morrell - steward of the Twitter account 'Adventures of Mr Muppet and first mate, Oscar' - documented the curious event while sailing near Auckland's Kawau Island on Thursday.

The biblical scene got "weirder and weirder" as she got closer to the shore near Bostaquet Bay.

"About midway through our journey from Algies Bay to Kawau Island we saw the odd fish bobbing on the surface," Morrell told Stuff.

"We passed Beehive Island and it got worse, then it just got thicker and thicker as we approached Bostaquet Bay."

There were thousands, she said, and they started bobbing into the boat.

"Nothing was feeding on them in the sea; no sharks, no seals, no birds," Morrell said.

"It was very strange, like a plague or something."

"They did smell on the beach and I turned a few over but could not see any injuries," she said.

"The birds had picked the eyes out but the flesh had not been touched."

Morrell, who sold up three years ago to go on an adventure after ill health, has not seen anything similar on past voyages.

"It was also very weird there was only one type of fish - mackerel."

The Department of Conservation and Ministry for Primary Industries were looking into the unusual event.

Meanwhile, Mr Muppet and first mate, Oscar, were continuing on their adventures.