A woman has been charged after allegedly leaving a baby boy locked in a hot car outside a hotel in Melbourne's southwest, as police investigate whether she was playing pokies inside.

Police were called to The Brook hotel on Sneydes Rd in Pt Cook about 3.15pm on Wednesday following reports a child had been left in a car alone

Paramedics arrived and treated the boy before taking him to the Royal Children's Hospital in a critical condition.

The 14-month-old boy, who was allegedly left in the vehicle for up to five hours, was on Thursday still fighting for his life in hospital.

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His 32-year-old mother was charged overnight by the Westgate Sexual Offences and Child Abuse Investigation Team with negligently causing serious injury and reckless conduct endangering life.

The woman, from Gladstone Park in the city's northwest, was released on bail and is due before Melbourne Magistrates' Court on January 23.

If the boy does not survive, the investigation could be transferred to the homicide squad, the Herald Sun reports.

Alison Connell, 58, who lives across the road from the pub told the newspaper: "My mum called me and said there's been a child left in a car and it's in a pretty bad way. I was horrified."

Temperatures in Melbourne reached 36C on Wednesday.

Tests previously run by Ambulance Victoria found even on a 29C day the inside of a car can reach 44C in 10 minutes and 60C within 20 minutes.

Kidsafe Victoria says leaving children unattended in a hot car, even for a short time, "can be fatal".

"Children are particularly at risk because they can lose fluid quickly, become dehydrated and suffer from heatstroke," the organisation states on its website.

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"Every year in Australia, over 5000 children are rescued after being left unattended in a car."

It shared a video on Monday from Ambulance Victoria reminding people of the dangers of leaving children unattended in cars given the warm weather forecast across the state.

It poses the question: "You wouldn't leave your ice cream in a hot car. Would you leave a child? Hot cars kill."