The death of a three-year-old boy who had been left locked in a hot car in Glenfield in Sydney's southwest on Thursday, sparked outrage by some on social media.

Campbelltown, near Glenfield, hit a top temperature of 33.4C at 4.30pm, Bureau of Meteorology data revealed.

This sort of tragedy is very rare, Kidsafe executive officer Christine Erskine said, but there are 5000 "near misses" where children have been rescued from a hot car.

"It almost can happen to anybody," she said.

"A lot of people would understand how this could happen."

Ms Erskine said while some cars lock automatically and the child's carer notices straight away, children are often 'left' when there's a change in circumstances.

"It can happen when the mother or the grandmother takes the child and this time someone else takes it," she said.

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"A change of routine or a change of circumstances is the riskiest time."

Children are often left in the vehicle while parents are paying for petrol, but Ms Erskine said they should never be left when you will be out of sight, such as going into a shopping centre.

"Little bodies heat up very quickly," she said.

She urged people to leave their valuables - keys, laptop, wallet - in the back of the vehicle where their child is so they are prompted to retrieve them and their child before leaving their vehicle.

The 45-year-old father of the three-year-old boy has now been released from police custody, while officers continued investigating the death.

The vehicle's owner, the child's father, broke the car's window to retrieve the boy before sitting nearby wailing and banging his head, the ABC reported.