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The whale was spotted near the entrance to the harbour travelling north, and the skipper moved to get a closer look.
George Waghorn, owner of cruise company Akaroa Dolphins, told the Herald it was an exciting moment for all on board.
"We were heading back into the harbour and I saw it in the back of my eye. It looked like a big boat in the distance but I saw its blowhole and knew what it was."
Waghorn said it is a reasonably rare occurrence that seems to be more frequent this year than in previous years.
He said it was a treat for the many Australian passengers on board.
"It was a great surprise for them. They loved it, they all got out the front of the boat, lots of cameras were out."
Akaroa Dolphins uses specially-trained dogs to help locate the Hector's Dolphin, one of the rarest in the world.
Onboard on Saturday was Albie the dog, Waghorn said he was not too keen on the big mammal.
"He wasn't too sure about it. Was giving it a bit of a look from the boat."
The whale spent about 20 minutes breaching in the water, Waghorn said.
-By Devon Bolger