"To be fair, everyone was a bit worried," Barkley, a former All White from 1986 to 1991, said.
"Then we saw a woman - looking a bit tired - being stretchered off, and everyone was feeling sad thinking maybe she was really ill.
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"But then coming behind was a guy clutching a shawl, and we were like, 'Is that a baby?'
"Someone asked him if it was, and he said yes. He was smiling so it seemed like everything was all right."
Barkley said there were about 200 people waiting in line and as soon as the news spread they all began to cheer and applaud the new parents.
"Everyone was smiling, there were cheers, and tears. I've never seen anything like it, born at sea, in the harbour."
Barkley heard staff say the baby was born as the ferry approached the harbour.
The news drew dozens of congratulatory comments on social media.
"Congratulations mum and dad. Destined to be a boatie," one person said.
Another suggested a few perks for the newborn.
"Free tickets for life [for] the baby," they said.
A St John Ambulance spokeswoman confirmed a woman had given birth on the ferry.
They received a call at 7.39am and met the ferry at the Downtown ferry terminal as it docked.
The baby had been born before staff arrived, and they transported the mother and baby from the ferry to Auckland City Hospital arriving at 8.35am via ambulance.
An Auckland District Health Board spokeswoman said the mother and her baby were "doing well" and in the birthcare unit.
Fullers360 fleet operations officer Megan Watson said the baby was born aboard their vessel Korora on the 7am sailing from Waiheke.
Their crew were on hand to assist the woman, who was with her midwife, during and right up to the birth.
"We would like to congratulate the mother on her new arrival and will be reaching out to extend our congratulations to her and the family," Watson said.
"Our people are well trained to provide assistance and support in situations like this, and we are very proud of how they responded quickly and supported our customer and her support group during this time.
"It will be a sailing to remember for our crew."