"The school is just around the corner, she would have been out for two to three minutes and came back," the grandmother said.
"And then her daughter was awake and had walked outside. She thought her brother would have seen her but he was fast asleep."
Her brother works early morning shifts, and was tired and went to sleep when he got home, the grandmother said.
He slept through the commotion of the child waking up, and the woman who found her knocking on the door to try to see if anyone was home.
However, she acknowledged that her daughter's decision was not the best one - and said she had given her a telling-off when she saw the video on Facebook.
"I told her off and I got my granddaughter," she told the Herald.
"I can see where she went wrong, that she should have woken up her brother to watch her daughter."
Her daughter told her she was certain the child was safe, she said.
"She said, 'I knew [my brother] was home, and she was safe with him'.
"She would never leave her alone."
Her daughter was "devastated", she said, but acknowledged that she "didn't think".
She said: "I know my daughter, she loves her children.
"She would be sick if she doesn't have them for one night [if they stay with grandparents, for example]."
However, she said people's reaction to the video had upset her daughter, who was now struggling to cope with criticism of her decision.
"They don't know what she's going through right now," she said.
She added: "She's so devastated, very, very upset about the whole thing."
In the video, the little girl is heard crying for her mother. The young woman who posted the video, approaches the child on the footpath and asks where she lives.
She then follows the toddler across the road to a house where they walk upstairs and through an open back door. The woman knocks and calls out, but no one answers and the child goes into a room.
It is not clear where or when the video was taken but it was posted on Facebook on Monday afternoon by a young mum who lives in Otara.
The video has been watched 115,000 times by 11am today.
The grandmother told the Herald her granddaughter was safe, and had slept well overnight.
Police today said they had identified the child and her mother.
Officers had spoken to the mother to establish what had happened, Counties Manukau Inspector Wendy Spiller said.
"The child's welfare remains the priority and police are working with the family and the Ministry for Vulnerable Children, Oranga Tamariki.
"We encourage anyone with concerns about a child's welfare to contact police."
Under the Summary Offences Act, a parent or guardian who leaves a child under 14 without reasonable supervision and care is subject to a fine of up to $2000.
"Police strongly encourage anyone with any concerns about a child's welfare to contact police," a police spokeswoman said. "If it's an immediate issue, don't hesitate to call 111."
Members of the public could also refer concerns to the Ministry for Vulnerable Children, she said.