Brad Nicholls and Emmy-Rose Curtis both work for whale watching companies and have years of experience being around whales and other ocean wildlife. That afternoon they had gone out on a friend's boat off South Stradbroke Island just south of Brisbane for fun and to practice with some new video equipment.
"We were just playing around underwater with the GoPros and all of a sudden there was something much larger than a turtle there," Curtis told ABC. "We had no fins, no goggles; it was certainly not anything anticipated."
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For more than an hour, the mother and calf would approach the swimmers before diving deep into the water only to surface once again.
"Their movements were so gentle and so slow, almost like they were like 'Oh, I don't want to hurt my new friends,'" Curtis said. "In 10 years, I have never seen this type of behavior for whales with a boat, let alone with us in the water."
"We kept getting out of the water and they would be coming up to the boat and spyhopping (poking their heads out of the water vertically) as if to say, 'Where'd you go? What are you doing? Come back and play,'" Curtis told Brisbane's 7 News.
Curtis said she was initially hesitant to post her footage for fear that others would go out and try to create the experience but later decided it was too incredible not to share.
"It was like hanging out with Mother Nature," she said. "My heart was so full of love at that moment."
Adult humpback whales can be 48-62 feet long and weigh up to 80,000 pounds.
Would you hop in the water to swim with these majestic creatures? Let us know in the comments below.