Cedric Horner received an early morning surprise on Wednesday when he spotted what looked like a fin in the water between Wellington's Houghton and Island Bays.
On closer inspection, he realised the fin in the water was actually a seal flipper.
In the video, which was originally posted to public community Facebook page Vic Deals, the seal can be seen lifting one flipper up and down, and appears to be lying on its side.
"We thought it might have been injured as it was just lolling about," Cedric told Newshub.
Cedric became concerned about the seal when it appeared to be motionless for more than 30 minutes.
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But the Department of Conservation (DoC) told Newshub swimming with its flipper up in the air is the seal "just thermo-regulating."
"The flipper has blood vessels close to the surface so they hold them up when they are in the water sometimes to warm the blood, which helps them to maintain their body temperature."
DoC says this isn't a sign the seal is stranded.
"Most seals reported as injured or sick are simply resting."
DoC asks the public to call them if they see a seal tangled in a net or rope, or see a seal in danger from dogs, vehicles or humans.