Michelle Nguyen's Supreme Court trial was expected to begin this week, but Justice Betty King heard on Thursday that the 21-year-old had decided to plead guilty over the death of daughter Thy Tran on November 5 last year.

Thy was 10 months old when she died after Nguyen left her sleeping in the vehicle while it was parked in direct sunlight, and with the doors and windows closed.

The temperature at midday on the day in Glenroy, where Thy died, was 26.3 degrees and was 28.2 degrees at 3pm.

A neighbour had earlier reminded Nguyen about the baby being in the car when she heard her crying, a previous court hearing was told.

Nguyen had taken Thy and her other daughter, 2, for a drive to get them to sleep.

When they returned home about 30 minutes later, Nguyen removed Thao but left Thy sleeping in the car in the driveway.

Nguyen returned to the car about 15 minutes later, about 12.15pm, to check Thy, and then returned inside, intending to come back in 15 minutes.

Instead, she went back inside and fell asleep in a chair while Thy remained in the vehicle.

Nguyen woke in a panic about 2pm and realised her daughter was still in the vehicle, but when she removed her the child was unresponsive.

Paramedics were unable to revive the baby, whose temperature was found to be 41.5 degrees Celsius.

Justice King extended Nguyen's bail and ordered her to re-appear for a pre-sentence plea hearing in February.