Mr Preston had given Carter to his mother to carry downstairs from the fourth-floor bedroom while he helped Ms Nguyen, who was recovering from her caesarean operation.
Ms Ferne, 68, and Carter suffered head injuries, and hers were fatal.
The family rushed to their aid and Mr Preston said he was faced with a heartbreaking decision - who to help first.
Ms Ferne was bleeding heavily but was still breathing.
Carter was not, and Mr Preston gave his tiny son mouth-to-mouth resuscitation.
He then carried his mother into the street while Ms Nguyen took their son.
There they commandeered a taxi and took the pair to hospital.
Ms Ferne had no heartbeat when they arrived.
Carter was taken to intensive care with bleeding on his brain.
Speaking from Ho Chi Minh City yesterday, Mr Preston described the shock of losing his mother days after celebrating the birth of his first child.
"It's like being on the biggest high of my life, really - a newborn son, a new country and establishing a business over here, and then my mother comes and visits and she's here in time to witness and be part of the birth of my son.
"And then to have it ... become the worst nightmare you could ever imagine, it's something that just takes a long time to comprehend."
Mr Preston said he could not have asked for a more loving and caring mother.
"She was very intelligent, very caring in the community, highly respected among her peers and her community wherever she lived," he said.
Carter remains in a critical condition in Ho Chi Minh Children'sHospital but is showing signs of improvement.
He was looking "very healthy" and was focusing with his eyes, responding when the family sang to him, Mr Preston said.
"We do have high hopes at this stage."
Mr Preston and his partner, who is Vietnamese, moved to Ho Chi Minh City in June to establish a plastics-exporting business.
His brother, Sean Preston, said his mother had visited grandchildren in Singapore before going on to Vietnam.
"I know in the few days she had with Phil she was absolutely having the time of her life. She was really full of life, full of energy and full of love."
Her eldest son, Stefan, who lives in Auckland, said his mother lived her life entirely for other people and had very close friends throughout New Zealand.
She grew up in Waiouru, went to school in Ohakune and had lived in Coromandel, Christchurch and Auckland. In recent years, she had lived on the North Shore.
Her funeral would be held on the North Shore on September 14.
The family have set up a trust fund to ensure that "something good comes from this tragedy and in the memory of our mum", Stefan Preston said.
The trust would contribute towards new equipment at the Ho Chi Minh Children's Hospital.
The hospital was now so short of essential equipment that one family were taking shifts to constantly hand-pump a ventilator that was keeping their child alive.
Said Stefan Preston: "She's never really travelled to a developing country before and she just couldn't believe the poverty, and just wanted to help, and that's the best thing we could think of to honour her memory."
MEMORIAL TRUST
Donations to the Julie Ferne Memorial Trust can be made to ASB account 12-3198-0065427-00
