The edited clip shows New Zealand charter boat skipper Joe Brogan, 42, swim to the aid of a distressed baby humpback entangled in a lobster trap line.
The whale was spotted by Mr Brogan and his crew mates last week.
He anchored near the calf, before bravely jumping in to cut the rope tied around the whale’s tail.
Mr Brogan said even though he knew there could be sharks about, there was no going back.
“We knew that this was a meal waiting to happen but once I saw the calf I knew I would never forgive myself if I didn’t try to save it,” he said.
“We watched the calf for about 15 minutes and when it became clear it wasn’t getting itself free, I dove in without a second thought.
“Once I was in there, there was no way I was getting out until I released it and once I cut the rope, it loosened from around his tale and definitely would of fallen off on its own.”
The freed calf, apparently unharmed, spouted water from its blowhole before it disappeared beneath the waves.
Mr Brogan’s heroic actions were caught on film by his crew and the video has gone viral since it was uploaded.
Last night the video had been watched 50,000 times on YouTube and has featured on TV news bulletins and websites across Australia, New Zealand, Britain and the US.
Mr Brogan said the reaction to the clip was overwhelming but he still did not think he was any sort of hero.
“I’m not a hero I was just doing what anyone else who saw something like that would have done,” he said.
“The reaction to the clip has been amazing but I’m just happy the calf was saved.”
Mr Brogan has posted online he is keen to sell the full 19-minute version to the highest bidder with the money to be donated to charity.