An early morning house fire in Wollongong's northern suburbs that sparked an emergency response has exposed a horrifying case of child neglect that almost claimed the lives of three siblings.

The Mercury can exclusively reveal the two younger children, a two-year-old girl and four-month old baby boy, were left in the care of their 12-year-old sister while their mother drove to a neighbouring suburb to stay with her new boyfriend.

The mother, who cannot be named for legal reasons, left the children alone in the house about 9pm on Saturday, March 2, with a request that a friend who lived about 100 metres away check on them periodically.

The mother told police she planned on returning about 6am on the Monday - some 33 hours later - to get the oldest child ready for school.

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"When the accused left her children they were not under the supervision of any adult and had to fend for themselves," police said in documents tendered to Wollongong Local Court said.

"This resulted in the 12-year-old child being responsible for her two-year-old sister and four-month-old brother. She was expected to cook, clean and care for her younger siblings, which also included feeding and changing the baby's nappy."

The court documents said the near-tragedy began to unfold around 10.30pm when the girl started making dinner but fell asleep while the meal was still on the stove.

The cook top caught alight and the fire soon spread around the kitchen, melting plastic items and releasing noxious smoke throughout the house, which each of the children inhaled.

The home's smoke alarm went off just before 2am and woke the neighbours, who saw the fire through a window and immediately went to the children's aid.

All three were safely removed from the house but had black soot marks visible around their mouths and noses. They were taken to hospital and treated for carbon monoxide poisoning.

Police repeatedly attempted to contact the mother but to no avail. When she did turn up several hours later, the 37-year-old said her phone battery had gone flat and she'd only received their messages after charging it.

She admitted leaving the children alone on the house, saying she'd only been with her boyfriend for a few weeks and didn't want to introduce him to her children in case things didn't work out.

Police charged the woman with three counts of failing to provide for a charge causing danger of death.

She pleaded guilty to the charges during a recent court appearance. The case was adjourned to September 17 for a psychological report to be prepared for sentencing.

The children are no longer in the woman's care.