One of five children left in a van while their parents gambled at SkyCity casino was up against the window "screaming and banging", said a woman who found them.
She and her partner, returning to their car on Sunday morning, could hear the children screaming in the casino carpark and alerted security and police, who broke into the van.
The youngest was aged 5 months and the oldest was just 8.
Surveillance cameras showed their parents left them, to play pokies, 45 minutes earlier.
The woman said they were clearly distressed. "I could see this little toddler up against the window screaming and banging," she told Close Up.
She could see a little boy trying to calm a baby. "He was looking at me [shaking his head], 'No, no, no', and he had a cellphone in his hand."
The boy looked as though he was shaking the baby too hard. When police arrived, they found black plastic covering a back window. An officer cut it with a knife because the children would not open the locked doors and were "screaming their heads off".
The woman said a 15-month-old was holding an empty bottle and it was "stuffy, boiling" inside, which made her even angrier.
"How the two of them as parents of five children decided between them it was okay to leave these children in a public car park and go gambling ... Surely it was against human nature as well to leave children in an abandoned dark, boiling hot van."
She said it was "sick" they could be regular users of the casino to get a loyalty card to pay for parking.
This latest incident is just the tip of the iceberg and the problem of parents abandoning their children at casinos is getting worse.
Since 2010, 231 children have been left alone by their parents at casinos around the country. Internal Affairs spokesman Trevor Henry said that in 2010, 84 children were found alone at casinos in 51 incidents. Of those, 11 were in car parks.
Last year, there were 59 incidents involving 101 children, with 14 being found in car parks.
Already this year there have been 25 incidents involving 46 children, including six in car parks. Most cases were in Auckland, Mr Henry said.
Auckland City police spokeswoman Noreen Hegarty said: "What our officers encountered were distressed children in a situation they should never be placed in."
Anti-gambling commentators and politicians say Sunday's incident - considered the worst in SkyCity's 16-year history - is further proof of the child neglect often seen at casinos.
Yesterday, Child, Youth and Family confirmed the family were known to staff because of violence between the parents and their "parenting skills".
"We were already working with this family and had an agreed plan in place to address concerns these children may be at risk," said northern regional director Grant Bennett.
The agency had increased its monitoring of the family and the support around them in recent weeks.
"This distressing and unacceptable event shows these parents have failed to stick to agreements they have made to ensure their children are safe."
The children are being cared for together by CYF caregivers and their mother is allowed supervised access.
Mr Bennett said that although there were no concerns for the children physically, they would receive health checks and whatever counselling they needed.
SkyCity general counsel Peter Treacy said the incident was upsetting for everyone concerned.
He said similar cases happened occasionally, but they were rare, and this was "without doubt the worst" in the company's 16 years in Auckland.
"SkyCity has zero tolerance for children being left without adult supervision anywhere on any of its premises."
The casino said its car parks were covered by surveillance cameras and frequently patrolled by staff, who were alert to the risk.
Problem Gambling Foundation chief executive Graeme Ramsay said the 25 incidents so far this year were "shocking figures".
"We know that gambling has a huge social cost. One of the big areas of that is child abuse and neglect and that's on top of the other problems like crime and mental health and domestic violence."
Ms Hegarty said it could be several days before the police made any decisions about any charges the parents could face.
Green Party gambling spokeswoman Denise Roche said the social misery problem gambling causes would escalate if the Government went ahead with its plan to let SkyCity operate more poker machines.