A group of sightseers signed on for a whale-watching cruise off Brier Island to perhaps catch a sight of a humpback whale, but nobody expected to have a close up adventure with one of the most powerful and mysterious predators in the sea.

It was already a great day for whale watching they'd seen three species, but then:

"I heard our captain yell, 'Holy smokes I think it is an orca,'" said Tania Campbell, who works for Welcome Aboard Whale Watching Tours.

"This was something distinct we had never seen anything like it before."

At first the tour could just see a fin on a far horizon, about half a mile. But as the boat approached "it got exciting really fast."

When the whale breached, the group could see the white, black and grey shape of the whale.

The killer whale is not a species common in east coast waters. (Tania Taylor-Campbell)

Then it would disappear and breach again.

"When an orca surfaces you see the tip of their dorsal and then it just keeps getting bigger and bigger until their head breaks the surface as well," Campbell said.

"One of the times we didn't see him until he broke the surface, and he was right along side us. Yeah, very close."

The killer whale is not a species common in east coast waters.

An orca, believed to be the same one, was spotted off Grand Manan Island off New Brunswick last year.