A Sydney father thought he'd dropped his son off at daycare. He'd left him in the car.

A Sydney father who thought he’d dropped his son at school on Thursday morning before driving to work was shocked to discover he’d left him in the locked car.

His three-year-old son was found dehydrated and visibly distressed inside a people mover parked outside a unit block on Powell Street in Neutral Bay about 11.15am.

After neighbours saw the boy moving and called authorities, officers broke the window of the car to rescue the toddler. It’s believed he was inside the car alone for three hours.

Thankfully, the boy suffered only mild dehydration, and after being sent to Royal North Shore Hospital for observation has been allowed to go home.

Speaking to reporters after arriving home to find his car had been impounded and a note from police, the father seemed understandably stressed and confused by the situation as he explained what had happened.

"We didn't sleep ... it was a rough night," he said.

"I went to drop off my kid at the school, he fell asleep in the car. I thought I had dropped him off and I went straight to work."

It's an awful situation, but it could have been much worse. Children die in hot cars, as a Victorian mother discovered in February 2015.

Kyneton mum Romy Zunde was, just as this Sydney father described, sleep-deprived when she thought she dropped her 22-month-old son at daycare one hot February morning.

It was only when she arrived at the childcare centre that afternoon that she discovered she hadn't. Instead, her son, Noah, had been sitting in the hot car that entire day and tragically, died of heatstroke.

The parents are not alone in their experience of unintentionally leaving their child in their car. In fact, the phenomenon has tentatively been named "forgotten baby syndrome".

International memory expert Dr David Diamond has given evidence in many cases of ‘forgotten baby syndrome’, including that of Bendigo mother Jayde Poole, who was acquitted of the manslaughter of her baby girl in 2012 after falling victim to the same memory lapse.

He said the neurological explanation for the phenomenon involves two parts of the brain competing and causing a memory failure.

Dr Diamond said the habit-forming part of the brain can override the multi-tasking and fact-based part of the brain, resulting in us forgetting to do something we had intended to, like buy milk on the way home from work, or drop a child off at daycare.

He said three factors, fatigue, stress and a change of routine create the perfect storm for forgotten baby syndrome.

“I don’t consider this poor parenting,” Dr Diamond said.

“It’s just a matter of chance that some people happen to be caught up in this combination of events that results in good parents forgetting their children, so I don’t categorise it as bad parenting.

“If you have no awareness, then what you have is a tragedy, but not a crime.”

Meanwhile, some parents intentionally leave their children in cars, which is what police and paramedics are warning against this summer. They warn that while parents might intend for their stay inside to be quick, it doesn't take long for the car to heat up to dangerous temperatures.

NSW Police say even on a mild day, the temperature inside a car can reach upwards of 40C. And on a hot day, according to NSW Ambulance Chief Inspector Brian Parsell, the interior could reach nearly 80 degrees within minutes.

"The situation can quickly cause damage to body cells leading to unconsciousness, shock, organ failure and death. Even in milder temperatures, children and babies can get sick very quickly," he said.

-With AAP