Floral tributes have been left outside the Logan home of two toddlers who died on Saturday in a hot car, with their mother in the watch house due to appear in court today.

Kerri-Ann Conley has been charged with murdering her daughters Darcey and Chloe-Ann, whose bodies were found in a black car at the front of their Waterford West home, south of Brisbane about 1:30pm on Saturday.

Kerri-Ann Conley has been charged with the murder of her two daughters. ( Facebook )

Temperatures reached 31 degrees Celsius on Saturday and police said the girls, aged one and two, had been in the car for a number of hours.

Detective Inspector Mark White said paramedics tried to revive the sisters, but they died at the scene.

He said they had been exposed to extreme heat.

A woman lays a bunch of flowers outside the home, after the children were found unresponsive in the black car. ( ABC News: Rachel Riga )

A neighbour, who did not want to be named, spoke to the media outside her house.

"They were always out in the yard playing. They were always happy little girls, that's for sure," she said.

"They were always gorgeous little girls, always dressed beautifully and always so happy, you could always hear them laughing, they were just really happy little girls."

People have started arriving at a makeshift memorial at the Logan house to lay flowers and toys for the girls.

A visibly upset woman arrived with her daughter and grandchild.

This woman, unnamed, arrived with her daughter and granddaughter to lay flowers. ( ABC News: Rachel Riga )

"Two innocent kids have done nothing to be locked in a car. I can't comprehend it," she said.

"I just can't comprehend how that could happen.

"We didn't know them and I am just devastated."

Local resident Sarah Boyle said she was shocked when she realised she knew the family.

"Just really heartbroken to hear her name come up this morning on the news because I didn't I honestly think that I would know the family, but I did," she said.

Temperatures rose to more than 30C on Saturday with the toddlers exposed to extreme heat inside the car. ( ABC News: Rachel Riga )

Ms Boyle spoke of how she had met one of the children before.

"She was probably only six months but she was a beautiful little thing, just sad to know that her and her sister are now dead," she said.

Another woman, with two children, laid two stuffed toys outside the house.

"It's just an awful tragedy. They were just babies," she said.

Their mother spent the night in the Brisbane watch house and will appear in the Brisbane Magistrates Court today, charged with two counts of murder.

The home remains a crime scene and police are appealing for information as they try to work out a time frame of events and exactly how long the children were in the car.

Detective Inspector White said authorities involved in the investigation would be cared for if they became distressed.

"It's tragic," he said.

"We are putting a lot of emphasis on supporting our people because it is a very difficult job that emergency services do generally and one of our priorities, in addition to conducting the investigation, is the wellbeing of our staff."