Police are investigating after a video was posted to Facebook showing an upset toddler dressed only in underwear wandering near a road.

The young child is heard crying: "Mummy".

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.

She found the child walking on the road crying, she said. In the video she is seen approaching the child on the footpath and asking where she lives.

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She then follows the toddler across the road to a house where they walk up stairs and through an open back door. The woman knocks and calls out but no one answers and the child goes into a room.

Soon after a woman arrives at the house in her car. She thanks the woman who found the child and says she had to leave because she was running late to do her cleaning.

"I was hard out beeping, like, 'Are you coming'," but the child said no," she says.

The video has gone viral on social media and had been watched by more than 89,000 people last night. Many outraged viewers called for the mother of the toddler to be referred to social welfare services or the police.

A woman who appears to be the mother has since commented on Facebook, saying her 25-year-old brother was inside and looking after the child but that "she always [does] this, walk out if she [doesn't] see me".

She later says she doesn't have a job and had gone to pick up another child and that the child was sleeping.

When the Herald contacted the woman she said the tot was actually her sister's child and didn't want to comment further.

The Herald is seeking comment from the woman who took the child home.

This morning the child's grandmother told the Herald her daughter was "very upset" by what had happened, and believed her child was safe when she left home.

She had been in the backyard cleaning and doing chores, when she lost track of time and realised she needed to leave to collect her 7-year-old son from school.

Her brother was asleep in the house, as was her daughter at the time, the grandmother said, so she decided to quickly drive to the school to collect her son while they slept.

"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 daughter "loves her kids very much", she said, and would never leave them alone.

Her grandchild was safe and well, and had slept well overnight, she said.

A police spokeswoman said there had been no reports about the child, but police were making inquiries. She referred the Herald to Section 10B of the Summary Offences Act.

Under the 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," the 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.