Passengers on board a nature cruise on Auckland's Hauraki Gulf were treated to a rare sight as scavenging sharks gorged themselves on a floating whale carcass.

The Auckland Whale and Dolphin Safari stumbled across the brutal encounter with 2m blue sharks feeding on the 17m-long bloated Bryde's whale north-east of Waiheke Island.

The Safari company's research and conservation officer Catherine Lea said it appeared to be a juvenile whale.

She said the encounter was only the second time in nearly two decades this had been observed by those running the safaris.

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"Although official confirmation has not yet been made, at 15-17m long, our initial thoughts, from what we could see, are that it appears to be either a juvenile fin whale or a Bryde's whale - the resident baleen species of whale seen year-round here in the Hauraki Gulf," said Lea.

"Although sad, observing a deceased whale carcass is a very rare and fascinating opportunity which we decided to make the most of. We were also able to observe several sharks, including 2m blue sharks, feeding on the whale as part of the natural circle of life."

Those on board were left amazed by the once-in-a-lifetime experience.

"When we spent two days travelling across the world to New Zealand we never thought we would see what we've seen today, so we will carry that memory with us forever," said visiting British tourist Natalie Payne.

Microbiologist Siouxsie Wiles said it was both a sad and amazing encounter.

"I remember the episode of Blue Planet about how much fat a whale carcass produces and to see that happen and sharks taking chunks out of it was fascinating."

Wiles said she watched as one shark swam on its back, around the tail, and then taking a bite.

She said the bloated and sunburnt whale, which was as big as the craft, had a pungent stench which was not pleasant downwind of the beast.

Those running the safari said it was the second time in less than a week a dead baleen whale had been found in the area.

Crew hoped it was not a result of ship-strike, with large vessels colliding with the sea giants.

"From PhD studies undertaken on board we know that there are less than 200 Bryde's whales in New Zealand waters.

"This already small population is particularly vulnerable to ship-strike by commercial ships as they spend the majority of their time in the top 10m of the water column.

"From what we could see there were no obvious signs of ship-strike to this whale but that does not necessarily eliminate it as a cause of death."

The safari company were in the process of sending all information to the Department of Conservation and local universities to help determine identification and cause of death.

LIVE FROM THE BOAT ⛴: Our afternoon safari has just had a RARE ENCOUNTER & left the scene of a deceased baleen whale....

Posted by Auckland Whale & Dolphin Safari on Monday, 22 January 2018