Whale watchers off the Kaikoura Coast were treated to a stunning display as a juvenile humpback whale breached repeatedly alongside their craft.

Images have been posted online of the barnacle-encrusted 10m mammal capturing the awe-inspiring moments it rears its giant body skyward out of the sea before crashing back under the water.

In one shot the juvenile humpback is shown seemingly standing upright with half its body out of the water, both long fins in a streamlined position.

Whale Watch Kaikoura marketing manager Lisa Bond said the breathtaking encounter was a rare treat for tourists and crew alike.

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"Everyone was buzzing, even the crew.

"Everyone was getting their camera out," Bond said.

The close-up encounter came as a complete surprise with the whale breaching alongside the boat at least 20 times.

It was taken on Saturday before conditions in the area deteriorated.

Bond said humpback whales normally appeared in the area between May and September as they migrated north to warmer tropical waters. However, a couple of younger humpbacks had been spotted in the region in the past month well ahead of timetable.