Australian police will continue to interview the mother of a five-month-old baby who died after being left alone in a car in Toowoomba in Queensland.

The mother left the girl in a parked car on Ramsay Street as she picked up her other children from St Thomas More's Catholic Primary School.

When the mother returned to the vehicle about 3.30pm (AEST), she discovered the baby dead inside her rear seat capsule.

It is not yet known how long the baby was left in the car, but it is believed school finished at 3pm.

Police had hoped a post-mortem examination would be conducted today, but that has now been pushed back to tomorrow.

Detectives interviewed the mother last night, and will speak to her again today along with other family members, a police spokeswoman said.

"The family is obviously distraught and is receiving support," the spokeswoman said.

Queensland Child Safety Minister Margaret Keech said her thoughts went out to the parents.

"The death of the child in Toowoomba is an absolute tragedy for all concerned," Ms Keech told reporters.

"My heart really does go out to the parents in particular.

"I'm advised by police that they are continuing their investigations and as a matter of course the department of child safety will investigate and support the police in their investigation."

Counsellors are at the school and teachers have been briefed on how to deal with students' questions.

AAP