Her hospital room had an ensuite and she was just metres away.
When she got out of the shower, the baby was gone.
Marsh admitted a charged of kidnapping and was jailed for 18 months in February.
After originally denying the same charge, Manutui accepted guilt and was today sentenced to three months community detention and 250 hours community work before the High Court at Auckland.
Two weeks into a relationship with Marsh, in January 2014, she told him she was pregnant, later providing scans of her sister's twin boys.
Despite a midwife telling the woman she was not pregnant, she told Manutui she was due the following month.
When the pair arrived at Middlemore Hospital, staff again told Marsh she was not pregnant but she sent her partner away while she supposedly asked them to check their files again.
About half an hour later, she walked out of the hospital with the baby girl and though Manutui confronted her she was adamant it was theirs.
"Despite a number of warning signs and inconsistencies in your partner's story, you took her and the baby home," Justice Mark Woolford said.
"You made no efforts to contact the hospital or the police."
When police knocked on their door at 4.40am the following morning, Manutui told them he was relieved as he knew something was not right.
He explained "that Ms Marsh did not appear to walk like a woman who had just given birth, and most problematically, that they were expecting twin boys and this was a baby girl, with very white skin despite his dark skin".
Justice Woolford accepted the woman went to great lengths to deceive her partner and his indiscretion was one of wilful blindness.
"Though your behaviour was unthinking and cavalier, it's not behaviour that warrants a sentence of imprisonment," he said.
Manutui's only previous conviction, for a breach of the Civil Aviation Act, was "very insignificant" according to his lawyer Nalesoni Tupou.
He was boarding a flight from Auckland to Nelson with "a couple of nice taros" in his bag, but when asked what was inside by a flight attendant he said they were bombs. He was subsequently fined.
Mr Tupou said his client was now in a new relationship.
