The toddler's father arrived late for pick-up on Monday to find Little Smarts childcare centre in Blaxland closed, and his child inside.

According to reports, the father called the police who broke into the centre. They found the child unharmed and in an air-conditioned room.

While many parents said on Facebook that they were appalled by the event, the boy's parents, seemed merely relieved the child was unharmed.

The parents, who refused to be identified, told Channel Seven news that they were happy with the childcare centre and continued to use it.

"The most important thing is our baby boy is safe," they said.

The owner of the centre said a "miscommunication" had resulted in the mishap and more checks would now be considered to avoid a repetition.

On Wednesday, a woman from Little Smarts told Fairfax Media the centre was being inundated with calls from the media and others about the incident.

"We are getting so many calls," she said, adding that it was hard to look after the little ones.

"I have five babies here," she said, hanging up.

A Department of Education spokesman said the incident would be fully investigated and the department would take appropriate compliance action.

Under guidelines by the Australian Children's Education and Care Quality Authority, all children must be "adequately supervised" when they are being cared for at an approved service.

Little Smarts, a long day-care centre that caters to babies, toddlers and preschoolers, was approved in 2009 and assessed against the National Quality Framework in 2014.

The regulatory authority must be notified within 24 hours if a child is mistakenly locked in or locked out of education or care service premises.

The authority's guidelines recommend procedures to ensure that children are accounted for at the end of the day.

Penalties for breaches are severe, including fines of up to $10,000.

Nesha O'Neil, the president of Australian Childcare Alliance, NSW, said best practice was for centres to combine a sign-in, sign-out form at the beginning and end of the day.

Parents who hadn't signed their child out should be called, and the centre shouldn't be closed until a pair of adults conducted a room-by-room check at the end of the day.

Considering there were more than 1.6 million children in childcare centres across Australia, Ms ONeil said these incidents rarely happened.

Often they happened when a series of things went wrong, such as a parent running late who had not rung, and a mix-up over the sign-in sign-out sheet. Often parents failed to sign the book, or consistently arrived late and failed to notify the centre, she said.

The guidelines say that, before closing a centre-based service, educators should look for visual cues that a child may still be on the premises (for example, a backpack left on a hook) and physically check each area, including sleeping areas, to ensure no child is accidently locked inside.

On Channel Seven's Facebook page, several childcare workers expressed their dismay.

"How terrible. As an an early childhood educator I'm embarrassed by this. There must have been some serious lack of team work with staff to forget they had a child asleep!" said one.

Another said: "As a child care worker for 25 years this is a worst nightmare. That's why there are procedures in place. Checking the role. Checking every room every sleep room !!"