"So I have been advised it cost the taxpayer no additional funding," she said.
Ardern added she would have been the first New Zealand leader in almost 50 years to miss the forum, aside from those who had been campaigning during an election cycle.
"I was damned if I did and damned if I didn't," she said, adding "At the end of the day, I am Prime Minister. I have a job to do."
Newstalk ZB's Barry Soper questioned whether the trip was necessary: "If Ardern decided not to go because of baby Neve, surely the family-focused Pacific leaders, more than any others, would have understood."
Ardern, 38, in June became just the second elected world leader in modern times to give birth while holding office, after Pakistan Prime Minister Benazir Bhutto gave birth to daughter Bakhtawar in 1990.
The leaders at the forum were signing a new security agreement called the Boe Declaration that identifies climate change as a major threat to security in the Pacific, since low-lying islands might cease to exist if sea levels keep rising.
The declaration also addresses crimes such as drug smuggling and illegal fishing that cross borders, as well as cybercrime and health concerns such as communicable diseases and pandemics. The agreement was the centerpiece of the three-day meeting.
Earlier today, Pacific fishing and community groups signed an agreement with the European Union to improve sustainable fishing and ocean governance in the region.
Under the Pacific-European Union Marine Partnership, the EU will provide €35 million ($41 million) and Sweden will provide €10 million ($12 million) over five years. The programme will provide direct assistance to regional organisations.
Tensions over China and refugees ran high at the forum after Nauru on Tuesday accused a Chinese official of bullying, and after Nauru police temporarily detained a New Zealand journalist who had been interviewing a refugee.
Nauru is home to more than 600 refugees who had tried to reach Australia by boat. The island's economy relies on the money Australia spends on keeping the refugees there.
Australia designed the policy of keeping boat refugees and asylum seekers far from its shores to deter more of them from trying to make the voyage, but many critics say the policy violates human rights.